ejabberd's mod_muc with XEP33 support

I have improved mod_muc (ejabberd implementation of XEP-0045: Multi-User Chat) to use XEP-0033: Extended Stanza Addressing in certain situations. This is still preliminary code, and includes some ugly code. But it seems to work.

Of course I followed the Guidelines for Server Active Multicasting on XEP33.

As previously mentioned on Multi User Chat and Extended Stanza Addressing, improving room presence broadcasts to use XEP-33 will be a little difficult. So, for now I concentrated on the easiest and potentially more beneficial improvements: message broadcasts.