Add keeping user's socket on server
Replace keeping user's socket by keeping user's ServerThread
Add equals method to User
Add checking user's uniqueness
Add getServerThreadMessenger method to ServerThread
Add getUserServerThread method to Server
Add refreshUsersList method to ClientFrame
Add template for the invitation to the game
Update javadoc
Add letsPlayRequested method to ServerThreadMessenger
Correct the mistake
Add NetworkClientMessengerListener
Finally! Synchronization usersLists works, but I don't know why.