Game is functional now. Performs moves slowly if you do them quickly in
succession. Random NullPointerExceptions occur, but they don't seem to
be linked to our methods (they stem from a GridWorld paint() method).

To-do list:
-Add documentation
-Highlight squares for available moves
-Fix jump chaining so that the same piece does not have to be clicked
twice
-Create actual piece displays
-Fix GridWorld GUI and its game messages
-Create menus for New Game, choose Human or Computer, etc.
-Networking