Code restructuring, assigning proper subsystems to each

- Drivers contain
    * donut/*
    * file/*
    * memory/*

 - Storage format contains
    * donut/*
      - GetObject() --> renamed to GetObjectReader()
      - Deleted stale objectwriter.go, renamed donutwriter.go to object_writer.go

Simplifying, and documenting codebase further

pkg/drivers/file/file_common.go

@@ -0,0 +1,89 @@
+/*
+ * Minimalist Object 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 file
+
+import (
+	"bufio"
+	"bytes"
+	"os"
+	"strings"
+	"sync"
+
+	"github.com/minio-io/minio/pkg/drivers"
+)
+
+// fileDriver - file local variables
+type fileDriver struct {
+	root string
+	lock *sync.Mutex
+}
+
+// fileMetadata - carries metadata about object
+type fileMetadata struct {
+	Md5sum      []byte
+	ContentType string
+}
+
+func appendUniq(slice []string, i string) []string {
+	for _, ele := range slice {
+		if ele == i {
+			return slice
+		}
+	}
+	return append(slice, i)
+}
+
+type bucketDir struct {
+	files map[string]os.FileInfo
+	root  string
+}
+
+func (p *bucketDir) getAllFiles(object string, fl os.FileInfo, err error) error {
+	if err != nil {
+		return err
+	}
+	if fl.Mode().IsRegular() {
+		if strings.HasSuffix(object, "$metadata") {
+			return nil
+		}
+		_p := strings.Split(object, p.root+"/")
+		if len(_p) > 1 {
+			p.files[_p[1]] = fl
+		}
+	}
+	return nil
+}
+
+func delimiter(object, delimiter string) string {
+	readBuffer := bytes.NewBufferString(object)
+	reader := bufio.NewReader(readBuffer)
+	stringReader := strings.NewReader(delimiter)
+	delimited, _ := stringReader.ReadByte()
+	delimitedStr, _ := reader.ReadString(delimited)
+	return delimitedStr
+}
+
+type byObjectKey []drivers.ObjectMetadata
+
+// Len
+func (b byObjectKey) Len() int { return len(b) }
+
+// Swap
+func (b byObjectKey) Swap(i, j int) { b[i], b[j] = b[j], b[i] }
+
+// Less
+func (b byObjectKey) Less(i, j int) bool { return b[i].Key < b[j].Key }