Support for multiple XMPP clients and ability to send to a room and user

Hey, the following features have been added to the gtalk submodule:
-  Support for different xmpp clients, including internal ones (though you may have to add to the etc/hosts file if the xmpp client is on a network behind a firewall)
-  Ability to send to rooms as well as users

Code added to errbit
- Spec tests for gtalk with new room ability
- Require 'xmpp4r/muc' for the room functionality
- Some additional fields to notifications services:
   service e.g talk.google.com
   service_url e.g http://google.com
   user_id same as room_id was originally - so actually applies to users
   room_id now actually applies to rooms

TODO:
   You may want to change the logo and name of gtalk to:
      XMPPService
   to make it clearer

If you find the code is not what your after please be gentle this is my first ever pull request

Thanks