XMPP: Skip messages coming from alien rooms #71

Even after #70 will fix #57, we'll still could have potential troubles if the bot receives a room message from any room other than the target one.

I can't currently imagine how it can happen if we have only one room monitored (well, it may happen if the admin log in under the bot account and join any alien room), but we'll need to filter messages from other rooms just in case.