Store persistent MUC room during creation

Make sure persistent rooms are stored to the database.  Without this
change, a room got lost if the 'persistent' flag was handed over to
mod_muc:create_room/5 and the server was then restartet before any
activity took place in that room.
Merge pull request #280 from weiss/store-persistent-muc-on-init

Store peristent MUC room during creation