Added function to handle username with textfield
Username is now checked and message given if it is already in use
Removed old socket.emit from handleplayerconnection
It is possible to pick an already existing name, as long as that player isn't currently active
Server remembers score in case a player reconnects with the same name
Merge branch 'username_screen' into development

Conflicts:
	src/server/ServerGame.js
	src/server/SocketHandler.js