fs: Break fs package to top-level and introduce ObjectAPI interface.

ObjectAPI interface brings in changes needed for XL ObjectAPI layer.

The new interface for any ObjectAPI layer is as below

```
// ObjectAPI interface.
type ObjectAPI interface {
        // Bucket resource API.
        DeleteBucket(bucket string) *probe.Error
        ListBuckets() ([]BucketInfo, *probe.Error)
        MakeBucket(bucket string) *probe.Error
        GetBucketInfo(bucket string) (BucketInfo, *probe.Error)

        // Bucket query API.
        ListObjects(bucket, prefix, marker, delimiter string, maxKeys int) (ListObjectsResult, *probe.Error)
        ListMultipartUploads(bucket string, resources BucketMultipartResourcesMetadata) (BucketMultipartResourcesMetadata, *probe.Error)

        // Object resource API.
        GetObject(bucket, object string, startOffset int64) (io.ReadCloser, *probe.Error)
        GetObjectInfo(bucket, object string) (ObjectInfo, *probe.Error)
        PutObject(bucket string, object string, size int64, data io.Reader, metadata map[string]string) (ObjectInfo, *probe.Error)
        DeleteObject(bucket, object string) *probe.Error

        // Object query API.
        NewMultipartUpload(bucket, object string) (string, *probe.Error)
        PutObjectPart(bucket, object, uploadID string, partID int, size int64, data io.Reader, md5Hex string) (string, *probe.Error)
        ListObjectParts(bucket, object string, resources ObjectResourcesMetadata) (ObjectResourcesMetadata, *probe.Error)
        CompleteMultipartUpload(bucket string, object string, uploadID string, parts []CompletePart) (ObjectInfo, *probe.Error)
        AbortMultipartUpload(bucket, object, uploadID string) *probe.Error
}
```

fs-bucket.go

@@ -0,0 +1,187 @@
+/*
+ * Minio Cloud Storage, (C) 2015 Minio, Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package main
+
+import (
+	"io/ioutil"
+	"os"
+	"path/filepath"
+	"strings"
+	"time"
+
+	"github.com/minio/minio/pkg/disk"
+	"github.com/minio/minio/pkg/probe"
+)
+
+/// Bucket Operations
+
+// DeleteBucket - delete a bucket.
+func (fs Filesystem) DeleteBucket(bucket string) *probe.Error {
+	// Verify bucket is valid.
+	if !IsValidBucketName(bucket) {
+		return probe.NewError(BucketNameInvalid{Bucket: bucket})
+	}
+	bucket = getActualBucketname(fs.path, bucket)
+	bucketDir := filepath.Join(fs.path, bucket)
+	if e := os.Remove(bucketDir); e != nil {
+		// Error if there was no bucket in the first place.
+		if os.IsNotExist(e) {
+			return probe.NewError(BucketNotFound{Bucket: bucket})
+		}
+		// On windows the string is slightly different, handle it here.
+		if strings.Contains(e.Error(), "directory is not empty") {
+			return probe.NewError(BucketNotEmpty{Bucket: bucket})
+		}
+		// Hopefully for all other operating systems, this is
+		// assumed to be consistent.
+		if strings.Contains(e.Error(), "directory not empty") {
+			return probe.NewError(BucketNotEmpty{Bucket: bucket})
+		}
+		return probe.NewError(e)
+	}
+	return nil
+}
+
+// BucketInfo - name and create date
+type BucketInfo struct {
+	Name    string
+	Created time.Time
+}
+
+// ListBuckets - Get service.
+func (fs Filesystem) ListBuckets() ([]BucketInfo, *probe.Error) {
+	files, e := ioutil.ReadDir(fs.path)
+	if e != nil {
+		return []BucketInfo{}, probe.NewError(e)
+	}
+	var metadataList []BucketInfo
+	for _, file := range files {
+		if !file.IsDir() {
+			// If not directory, ignore all file types.
+			continue
+		}
+		// If directories are found with odd names, skip them.
+		dirName := strings.ToLower(file.Name())
+		if !IsValidBucketName(dirName) {
+			continue
+		}
+		metadata := BucketInfo{
+			Name:    dirName,
+			Created: file.ModTime(),
+		}
+		metadataList = append(metadataList, metadata)
+	}
+	// Remove duplicated entries.
+	metadataList = removeDuplicateBuckets(metadataList)
+	return metadataList, nil
+}
+
+// removeDuplicateBuckets - remove duplicate buckets.
+func removeDuplicateBuckets(buckets []BucketInfo) []BucketInfo {
+	length := len(buckets) - 1
+	for i := 0; i < length; i++ {
+		for j := i + 1; j <= length; j++ {
+			if buckets[i].Name == buckets[j].Name {
+				if buckets[i].Created.Sub(buckets[j].Created) > 0 {
+					buckets[i] = buckets[length]
+				} else {
+					buckets[j] = buckets[length]
+				}
+				buckets = buckets[0:length]
+				length--
+				j--
+			}
+		}
+	}
+	return buckets
+}
+
+// MakeBucket - PUT Bucket
+func (fs Filesystem) MakeBucket(bucket string) *probe.Error {
+	di, err := disk.GetInfo(fs.path)
+	if err != nil {
+		return probe.NewError(err)
+	}
+
+	// Remove 5% from total space for cumulative disk space used for
+	// journalling, inodes etc.
+	availableDiskSpace := (float64(di.Free) / (float64(di.Total) - (0.05 * float64(di.Total)))) * 100
+	if int64(availableDiskSpace) <= fs.minFreeDisk {
+		return probe.NewError(RootPathFull{Path: fs.path})
+	}
+
+	// Verify if bucket is valid.
+	if !IsValidBucketName(bucket) {
+		return probe.NewError(BucketNameInvalid{Bucket: bucket})
+	}
+
+	bucket = getActualBucketname(fs.path, bucket)
+	bucketDir := filepath.Join(fs.path, bucket)
+	if _, e := os.Stat(bucketDir); e == nil {
+		return probe.NewError(BucketExists{Bucket: bucket})
+	}
+
+	// Make bucket.
+	if e := os.Mkdir(bucketDir, 0700); e != nil {
+		return probe.NewError(err)
+	}
+	return nil
+}
+
+// getActualBucketname - will convert incoming bucket names to
+// corresponding actual bucketnames on the backend in a platform
+// compatible way for all operating systems.
+func getActualBucketname(fsPath, bucket string) string {
+	fd, e := os.Open(fsPath)
+	if e != nil {
+		return bucket
+	}
+	buckets, e := fd.Readdirnames(-1)
+	if e != nil {
+		return bucket
+	}
+	for _, b := range buckets {
+		// Verify if lowercase version of the bucket is equal
+		// to the incoming bucket, then use the proper name.
+		if strings.ToLower(b) == bucket {
+			return b
+		}
+	}
+	return bucket
+}
+
+// GetBucketInfo - get bucket metadata.
+func (fs Filesystem) GetBucketInfo(bucket string) (BucketInfo, *probe.Error) {
+	if !IsValidBucketName(bucket) {
+		return BucketInfo{}, probe.NewError(BucketNameInvalid{Bucket: bucket})
+	}
+	bucket = getActualBucketname(fs.path, bucket)
+	// Get bucket path.
+	bucketDir := filepath.Join(fs.path, bucket)
+	fi, e := os.Stat(bucketDir)
+	if e != nil {
+		// Check if bucket exists.
+		if os.IsNotExist(e) {
+			return BucketInfo{}, probe.NewError(BucketNotFound{Bucket: bucket})
+		}
+		return BucketInfo{}, probe.NewError(e)
+	}
+	bucketMetadata := BucketInfo{}
+	bucketMetadata.Name = fi.Name()
+	bucketMetadata.Created = fi.ModTime()
+	return bucketMetadata, nil
+}