Probably a fix of issue #87

Actually, nothing really bad happens here, but the message itself is very confusing. After ~2 hours of investigation I found it very difficult to get rid of the message correctly. So, I decided to try just sending `UserJoin` from `JabberProtocol` to the message handler on joining the room. Hope it doesn't break anything. At least, on my local machine it works fine.
I think Horta should has a User affinity for now, so nobody can trick her to say admin's commands. In the future this should be fixed on another level.