MUC: send <actor nick="actor's nick"/> on kick/ban & affil/role change.

http://xmpp.org/extensions/xep-0045.html#ban

According to Example 113:
The service MUST also remove any banned users who are in the room by sending a presence stanza of type "unavailable" to each banned occupant, including status code 301 in the extended presence information, optionally along with the reason (if provided) and the **roomnick** or bare JID **of the user who initiated the ban**.