Jabber support

Hey there,

This is my first version of Jabber Support for Buildbot. It's mostly based on the work of  Saj Goonatilleke, who announced jabber support in 2011 on the buildbot-devel ML (see http://www.digipedia.pl/usenet/thread/15877/7748/).

The work uses the python-wokkel library for XMPP support.

My work included:
* Rebase code on top of the current code base
* Factor out the non-IRC bits out of words.IRC so we can reuse it in Jabber 
* Implement active client-to-server ping mechanism for Jabber
* Add documentation for the Jabber class

TODO:
* I am not sure if it's a good idea to add another module 'wokkel.py' in the buildbot tree (I just kept the approach of Saj)

That's it. We (KDAB) were using it for two weeks now, with one master instance controlling multiple jabber resources in different chats. It's working very well and reliable for us. Disconnects are handled as well.