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-backend-metadata.go

@@ -0,0 +1,57 @@
+/*
+ * 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 (
+	"github.com/minio/minio/pkg/probe"
+	"github.com/minio/minio/pkg/quick"
+)
+
+var multipartsMetadataPath string
+
+// SetFSMultipartsMetadataPath - set custom multiparts session
+// metadata path.
+func setFSMultipartsMetadataPath(metadataPath string) {
+	multipartsMetadataPath = metadataPath
+}
+
+// saveMultipartsSession - save multiparts
+func saveMultipartsSession(multiparts Multiparts) *probe.Error {
+	qc, err := quick.New(multiparts)
+	if err != nil {
+		return err.Trace()
+	}
+	if err := qc.Save(multipartsMetadataPath); err != nil {
+		return err.Trace()
+	}
+	return nil
+}
+
+// loadMultipartsSession load multipart session file
+func loadMultipartsSession() (*Multiparts, *probe.Error) {
+	multiparts := &Multiparts{}
+	multiparts.Version = "1"
+	multiparts.ActiveSession = make(map[string]*MultipartSession)
+	qc, err := quick.New(multiparts)
+	if err != nil {
+		return nil, err.Trace()
+	}
+	if err := qc.Load(multipartsMetadataPath); err != nil {
+		return nil, err.Trace()
+	}
+	return qc.Data().(*Multiparts), nil
+}