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-utils.go

@@ -0,0 +1,65 @@
+/*
+ * Minio Cloud Storage, (C) 2015, 2016 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 (
+	"regexp"
+	"unicode/utf8"
+)
+
+// validBucket regexp.
+var validBucket = regexp.MustCompile(`^[a-z0-9][a-z0-9\.\-]{1,61}[a-z0-9]$`)
+
+// IsValidBucketName verifies a bucket name in accordance with Amazon's
+// requirements. It must be 3-63 characters long, can contain dashes
+// and periods, but must begin and end with a lowercase letter or a number.
+// See: http://docs.aws.amazon.com/AmazonS3/latest/dev/BucketRestrictions.html
+func IsValidBucketName(bucket string) bool {
+	if len(bucket) < 3 || len(bucket) > 63 {
+		return false
+	}
+	if bucket[0] == '.' || bucket[len(bucket)-1] == '.' {
+		return false
+	}
+	return validBucket.MatchString(bucket)
+}
+
+// IsValidObjectName verifies an object name in accordance with Amazon's
+// requirements. It cannot exceed 1024 characters and must be a valid UTF8
+// string.
+// See: http://docs.aws.amazon.com/AmazonS3/latest/dev/UsingMetadata.html
+func IsValidObjectName(object string) bool {
+	if len(object) > 1024 || len(object) == 0 {
+		return false
+	}
+	if !utf8.ValidString(object) {
+		return false
+	}
+	return true
+}
+
+// IsValidObjectPrefix verifies whether the prefix is a valid object name.
+// Its valid to have a empty prefix.
+func IsValidObjectPrefix(object string) bool {
+	// Prefix can be empty.
+	if object == "" {
+		return true
+	}
+	// Verify if prefix is a valid object name.
+	return IsValidObjectName(object)
+
+}