Complete the diff generating functionality.

storage/s3.py

@@ -60,6 +60,13 @@ class S3Storage(Storage):
                 break
             yield buf
 
+    def stream_read_file(self, path):
+        path = self._init_path(path)
+        key = boto.s3.key.Key(self._s3_bucket, path)
+        if not key.exists():
+            raise IOError('No such key: \'{0}\''.format(path))
+        return key
+
     def stream_write(self, path, fp):
         # Minimum size of upload part size on S3 is 5MB
         buffer_size = 5 * 1024 * 1024