Start a websocket for user data

        https://github.com/binance-exchange/binance-official-api-docs/blob/master/user-data-stream.md

        :param callback: callback function to handle messages
        :type callback: function

        :returns: connection key string if successful, False otherwise

        Message Format - see Binance API docs for all types

def start_user_socket(self, callback):
        """Start a websocket for user data

        https://github.com/binance-exchange/binance-official-api-docs/blob/master/user-data-stream.md

        :param callback: callback function to handle messages
        :type callback: function

        :returns: connection key string if successful, False otherwise

        Message Format - see Binance API docs for all types
        """
        # Get the user listen key
        user_listen_key = self._client.stream_get_listen_key()
        # and start the socket with this specific key
        conn_key = self._start_user_socket(user_listen_key, callback)
        return conn_key