Publish a new event ``message`` to a ``channel``.

def publish_event(self, channel, event, message):
        '''Publish a new event ``message`` to a ``channel``.
        '''
        assert self.protocol is not None, "Protocol required"
        msg = {'event': event, 'channel': channel}
        if message:
            msg['data'] = message
        return self.publish(channel, msg)