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>
```

routers.go

@@ -149,6 +149,8 @@ func registerAPIHandlers(mux *router.Router, a storageAPI, w *webAPI) {
 	bucket.Methods("PUT").HandlerFunc(a.PutBucketHandler)
 	// HeadBucket
 	bucket.Methods("HEAD").HandlerFunc(a.HeadBucketHandler)
+	// DeleteMultipleObjects
+	bucket.Methods("POST").HandlerFunc(a.DeleteMultipleObjectsHandler)
 	// PostPolicy
 	bucket.Methods("POST").HandlerFunc(a.PostPolicyBucketHandler)
 	// DeleteBucket