U1db implementation

Docstring style comment again, we've been using:

    :return: PutSyncInfoResponse wrapped in an Identifier message.
    :rtype: zmq_transport.common.message_pb2.Identifier

Note the singular in `:return:` and the `:rtype:` identifying the type of the returned value.

Sorry for those, but we should maintain a standard throughout the code to make sure all our doc is generated correctly. :-)