Launches an app on the Chromecast.

            Will only launch if it is not currently running unless
            force_launch=True.

def launch_app(self, app_id, force_launch=False, callback_function=False):
        """ Launches an app on the Chromecast.

            Will only launch if it is not currently running unless
            force_launch=True. """

        if not force_launch and self.app_id is None:
            self.update_status(lambda response:
                               self._send_launch_message(app_id, force_launch,
                                                         callback_function))
        else:
            self._send_launch_message(app_id, force_launch, callback_function)