Added numpad support.

The game now fully works with all of the numpad directional keys. The player can now move diagonally by pressing only one key.

Issues:
* Diagonal movement is faster than orthogonal movement due to the grid representation used. Fixing this will require changes to the movement system as a whole.
* By holding two adjacent numpad keys the player will gain added speed. For example, holding 1 moves the player left at a rate of "speed" and down at a rate of "speed". Holding 2 as well moves the player left at a rate of "speed" and down at a rate of "2*speed".