def role_changed(self,user,old_role,new_role,stanza):
pass<EOL>
Called when a role of an user has been changed.

:Parameters:
    - `user`: the user (after update).
    - `old_role`: user's role before update.
    - `new_role`: user's role after update.
    - `stanza`: the stanza received.

:Types:
    - `user`: `MucRoomUser`
    - `old_role`: `unicode`
    - `new_role`: `unicode`
    - `stanza`: `pyxmpp.stanza.Stanza`