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.go

@@ -0,0 +1,141 @@
+/*
+ * 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 (
+	"os"
+	"path/filepath"
+	"sync"
+	"time"
+
+	"github.com/minio/minio/pkg/probe"
+)
+
+// ListObjectParams - list object params used for list object map
+type ListObjectParams struct {
+	bucket    string
+	delimiter string
+	marker    string
+	prefix    string
+}
+
+// Filesystem - local variables
+type Filesystem struct {
+	path               string
+	minFreeDisk        int64
+	rwLock             *sync.RWMutex
+	multiparts         *Multiparts
+	listObjectMap      map[ListObjectParams][]ObjectInfoChannel
+	listObjectMapMutex *sync.Mutex
+}
+
+func (fs *Filesystem) pushListObjectCh(params ListObjectParams, ch ObjectInfoChannel) {
+	fs.listObjectMapMutex.Lock()
+	defer fs.listObjectMapMutex.Unlock()
+
+	channels := []ObjectInfoChannel{ch}
+	if _, ok := fs.listObjectMap[params]; ok {
+		channels = append(fs.listObjectMap[params], ch)
+	}
+
+	fs.listObjectMap[params] = channels
+}
+
+func (fs *Filesystem) popListObjectCh(params ListObjectParams) *ObjectInfoChannel {
+	fs.listObjectMapMutex.Lock()
+	defer fs.listObjectMapMutex.Unlock()
+
+	if channels, ok := fs.listObjectMap[params]; ok {
+		for i, channel := range channels {
+			if !channel.IsTimedOut() {
+				chs := channels[i+1:]
+				if len(chs) > 0 {
+					fs.listObjectMap[params] = chs
+				} else {
+					delete(fs.listObjectMap, params)
+				}
+
+				return &channel
+			}
+		}
+
+		// As all channels are timed out, delete the map entry
+		delete(fs.listObjectMap, params)
+	}
+
+	return nil
+}
+
+// MultipartSession holds active session information
+type MultipartSession struct {
+	TotalParts int
+	ObjectName string
+	UploadID   string
+	Initiated  time.Time
+	Parts      []PartMetadata
+}
+
+// Multiparts collection of many parts
+type Multiparts struct {
+	Version       string                       `json:"version"`
+	ActiveSession map[string]*MultipartSession `json:"activeSessions"`
+}
+
+// newFS instantiate a new filesystem.
+func newFS(rootPath string) (ObjectAPI, *probe.Error) {
+	setFSMultipartsMetadataPath(filepath.Join(rootPath, "$multiparts-session.json"))
+
+	var err *probe.Error
+	// load multiparts session from disk
+	var multiparts *Multiparts
+	multiparts, err = loadMultipartsSession()
+	if err != nil {
+		if os.IsNotExist(err.ToGoError()) {
+			multiparts = &Multiparts{
+				Version:       "1",
+				ActiveSession: make(map[string]*MultipartSession),
+			}
+			if err = saveMultipartsSession(*multiparts); err != nil {
+				return Filesystem{}, err.Trace()
+			}
+		} else {
+			return Filesystem{}, err.Trace()
+		}
+	}
+
+	fs := &Filesystem{
+		rwLock: &sync.RWMutex{},
+	}
+	fs.path = rootPath
+	fs.multiparts = multiparts
+
+	/// Defaults
+
+	// Minium free disk required for i/o operations to succeed.
+	fs.minFreeDisk = 5
+
+	fs.listObjectMap = make(map[ListObjectParams][]ObjectInfoChannel)
+	fs.listObjectMapMutex = &sync.Mutex{}
+
+	// Return here.
+	return fs, nil
+}
+
+// GetRootPath - get root path.
+func (fs Filesystem) GetRootPath() string {
+	return fs.path
+}