KeyedProducer support sending multiple messages at once (mirror other producers)

For reasons I don't know, the KeyedProducer API is not equivalent to the SimpleProducer.

This PR adds `send_messages` method to support sending multiple messages at once.

A test has been added for the specific case.

I think we should remove send completely to make the producers more uniform but it represents an API break so I didn't include it here. 
I think I'd rather see something like this:

    def send_messages(self, topic, *msgs):
        msgs_by_key = groupby(...)
        for key, msgs in msgs_by_key.iteritems():
            self._send_messages(topic, key, *msgs)