Added new event: OnClientDisconnect
Added functionality to Model.ServerHandler - It listens for exceptions happening in each Task ClientHandler, then it sends an event through OnClientDisconnect

Changed EchoService to ClientHandler
Changed ClientHandler now accepts a User
Changed Client now contains a TcpClient property
Changed ViewModel.ServerViewModel to lock the ConnectedUsers collection property - Whether this work I don't really know but I imagine if two events happens concurrently, it wont mess with the collection.