def presence_changed(self,user,stanza):
pass<EOL>
Called whenever user's presence changes (includes nick, role or
affiliation changes).

:Parameters:
    - `user`: MucRoomUser object describing the user.
    - `stanza`: the stanza received.

:Types:
    - `user`: `MucRoomUser`
    - `stanza`: `pyxmpp.stanza.Stanza`