Added leveldata files, Sokoban level type, event handling

- Abstract level data file
- Mock level data file for testing
- Tiled XML level data file

- Sokoban level type

- Event handling added to the game loop, and a player move event added
to the level class
Remove MockLevel

At this point it only added the add_sprite method, and it's probably OK
to add that into the actual level class.
pygame in progress