Changed the way invalid input is handled to use a KeyListener rather than an ActionListener

Other changes:
 - Changed the layout of GameView to use Grid and Boarder rather than GridBag
 - Changed the layout of Client to Grid
 - Renamed "client.java" to "Client.java"
 - Starting to serialize GameView so games can be saved
 - Added a debugging funtion - press alt to view the numbers entered (as they appear in the ArrayList of JTextFields which is a field of the GameView class)