Updated XMPPMUC to include functionality for discovering rooms and services (issue #444)

Added additional functionality to XMPPMUC for discovering a MUC service ([XEP-0045 6.1](http://xmpp.org/extensions/xep-0045.html#disco-service)) and discovering rooms ([XEP-0045 6.3](http://xmpp.org/extensions/xep-0045.html#disco-rooms)).

This should handle issue #444 that I created yesterday.