Added a simple screen (to mainscreen) when no players are connected yet.
Updated username input when a player connects.

The name will be checked for availability (isNameAvailable returns
undefined if not present, so edited check accordingly).
NameList and playerNames were used throughout each other resulting in
errors. NameList is now being used.
Player.setName is commented in playerConnection.js, as player is
undefined at that point..
Merge branch 'server_fix_serverOverhaul' into serverOverhaul