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.
1. Due to concurrent nature of message passing, room actor may not receive this `UserJoined` message before it tries to find itself in participant list. (I am not sure what guarantees Akka gives to us, but I think that we simply shouldn't rely on it anyway.) So synchronous code in the actor constructor (or in the `preStart` method) is more reliable.
2. I think that in some cases the `nickname` is not the real nickname (in case of nick conflicts or another server stupidness). That's why I'm thinking about a solution dealing with Smack API.