api: Implement multiple objects Delete api - fixes #956

This API takes input XML input in following form.

```
<?xml version="1.0" encoding="UTF-8"?>
<Delete>
    <Quiet>true</Quiet>
    <Object>
         <Key>Key</Key>
    </Object>
    <Object>
         <Key>Key</Key>
    </Object>
    ...
</Delete>
```

and responds the list of successful deletes, list of errors
for all the deleted objects.

```
<?xml version="1.0" encoding="UTF-8"?>
<DeleteResult xmlns="http://s3.amazonaws.com/doc/2006-03-01/">
  <Deleted>
    <Key>sample1.txt</Key>
  </Deleted>
  <Error>
    <Key>sample2.txt</Key>
    <Code>AccessDenied</Code>
    <Message>Access Denied</Message>
  </Error>
</DeleteResult>
```

api-response.go

@@ -218,6 +218,24 @@ type CompleteMultipartUploadResponse struct {
 	ETag     string
 }
 
+// DeleteError structure.
+type DeleteError struct {
+	Code    string
+	Message string
+	Key     string
+}
+
+// DeleteObjectsResponse container for multiple object deletes.
+type DeleteObjectsResponse struct {
+	XMLName xml.Name `xml:"http://s3.amazonaws.com/doc/2006-03-01/ DeleteResult" json:"-"`
+
+	// Collection of all deleted objects
+	DeletedObjects []ObjectIdentifier `xml:"Deleted,omitempty"`
+
+	// Collection of errors deleting certain objects.
+	Errors []DeleteError `xml:"Error,omitempty"`
+}
+
 // getLocation get URL location.
 func getLocation(r *http.Request) string {
 	return r.URL.Path
@@ -414,6 +432,16 @@ func generateListMultipartUploadsResponse(bucket string, metadata fs.BucketMulti
 	return listMultipartUploadsResponse
 }
 
+// generate multi objects delete response.
+func generateMultiDeleteResponse(quiet bool, deletedObjects []ObjectIdentifier, errs []DeleteError) DeleteObjectsResponse {
+	deleteResp := DeleteObjectsResponse{}
+	if !quiet {
+		deleteResp.DeletedObjects = deletedObjects
+	}
+	deleteResp.Errors = errs
+	return deleteResp
+}
+
 // writeSuccessResponse write success headers and response if any.
 func writeSuccessResponse(w http.ResponseWriter, response []byte) {
 	setCommonHeaders(w)