Add the ability to send custom Xmpp.Element objects as messages

I wanted to selectively change the font-family and color of jabber messages sent by hubot, as outlined in the [xhtml-im integration set](http://xmpp.org/extensions/xep-0071.html) spec. 

This patch seems to work for my purposes, but as a node/javascript/coffeescript/hubot newb, I'm not particularly confident in the implementation. 

Criticism, advice, commentary of any kind, is greatly appreciated.