Add support to posting to an XMPP MUC room

This service will connect to an XMPP MUC (Multi User Chat) room and post github updates.

I've supported as many event types as there are example payloads for and will happily add additional events as the community picks up and starts to use the service.

I've mainly used the same messages from the IRC service with a couple of minor modifications and the addition of wiki events. It would be really great if these two services could share code to generate the events as it seems silly to duplicate effort.

A full test suite is included and no additional gems are required since `xmpp4r` was already available.
@kdaigle removed the commented options. I had hoped to support the full list of events but have settled with supporting the same as the IRC service. As I think I mentioned in a previous comment it would be good to push the message generation to another class and share between IM style notification systems (XMPP/IRC/hipchat/slack/gitter/etc).  But that can come later :)