ejabberd's C2S with XEP33 support

I have improved ejabberd's C2S code to use XEP-0033: Extended Stanza Addressing when the users login, logout or update presence.

This means that, when a user sends initial presence to login, or logout, or updates his presence, the server instead of sending a single packet to each contact on his roster, tries to send a packet to each destination server.

As usual, I've commited the change to ejabberd-modules SVN and the changes can be seen here: ejabberd_c2s.erl.

As previously with the MUC service, I followed the Guidelines for Server Active Multicasting on XEP33.