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.
And another one is that horta always detects itself as `User` (even if real affinity is `Admin`, for instance). I think we can currently live with that, so I'll accept the request. Thank you.