Register an on_recv callback

def on_recv(self, callback):
        '''
        Register an on_recv callback
        '''
        if callback is None:
            return self.message_client.on_recv(callback)

        @tornado.gen.coroutine
        def wrap_callback(body):
            if not isinstance(body, dict):
                # TODO: For some reason we need to decode here for things
                #       to work. Fix this.
                body = salt.utils.msgpack.loads(body)
                if six.PY3:
                    body = salt.transport.frame.decode_embedded_strs(body)
            ret = yield self._decode_payload(body)
            callback(ret)
        return self.message_client.on_recv(wrap_callback)