objectAPI: Fix object API interface, remove unnecessary structs.

ObjectAPI changes.
```
ListObjects(bucket, prefix, marker, delimiter string, maxKeys int) (ListObjectsInfo, *probe.Error)
ListMultipartUploads(bucket, objectPrefix, keyMarker, uploadIDMarker, delimiter string, maxUploads int) (ListMultipartsInfo, *probe.Error)
ListObjectParts(bucket, object, uploadID string, partNumberMarker, maxParts int) (ListPartsInfo, *probe.Error)
CompleteMultipartUpload(bucket string, object string, uploadID string, parts []completePart) (ObjectInfo, *probe.Error)
```

object-datatypes.go

@@ -0,0 +1,109 @@
+/*
+ * Minio Cloud Storage, (C) 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 "time"
+
+// BucketInfo - bucket name and create date
+type BucketInfo struct {
+	Name    string
+	Created time.Time
+}
+
+// ObjectInfo - object info.
+type ObjectInfo struct {
+	Bucket       string
+	Name         string
+	ModifiedTime time.Time
+	ContentType  string
+	MD5Sum       string
+	Size         int64
+	IsDir        bool
+	Err          error
+}
+
+// ListPartsInfo - various types of object resources.
+type ListPartsInfo struct {
+	Bucket               string
+	Object               string
+	UploadID             string
+	StorageClass         string
+	PartNumberMarker     int
+	NextPartNumberMarker int
+	MaxParts             int
+	IsTruncated          bool
+
+	Parts        []partInfo
+	EncodingType string
+}
+
+// ListMultipartsInfo - various types of bucket resources for inprogress multipart uploads.
+type ListMultipartsInfo struct {
+	KeyMarker          string
+	UploadIDMarker     string
+	NextKeyMarker      string
+	NextUploadIDMarker string
+	EncodingType       string
+	MaxUploads         int
+	IsTruncated        bool
+	Uploads            []uploadMetadata
+	Prefix             string
+	Delimiter          string
+	CommonPrefixes     []string
+}
+
+// ListObjectsInfo - container for list objects.
+type ListObjectsInfo struct {
+	IsTruncated bool
+	NextMarker  string
+	Objects     []ObjectInfo
+	Prefixes    []string
+}
+
+// partInfo - various types of individual part resources.
+type partInfo struct {
+	PartNumber   int
+	LastModified time.Time
+	ETag         string
+	Size         int64
+}
+
+// uploadMetadata container capturing metadata on in progress multipart upload in a given bucket
+type uploadMetadata struct {
+	Object       string
+	UploadID     string
+	StorageClass string
+	Initiated    time.Time
+}
+
+// completePart - completed part container.
+type completePart struct {
+	PartNumber int
+	ETag       string
+}
+
+// completedParts is a sortable interface for Part slice
+type completedParts []completePart
+
+func (a completedParts) Len() int           { return len(a) }
+func (a completedParts) Swap(i, j int)      { a[i], a[j] = a[j], a[i] }
+func (a completedParts) Less(i, j int) bool { return a[i].PartNumber < a[j].PartNumber }
+
+// completeMultipartUpload container for completing multipart upload
+type completeMultipartUpload struct {
+	Parts []completePart `xml:"Part"`
+}