Major update!
- TMXLoader is complete and seems to be fully functional. Use it to load
  TMX files into an easily-traversable object tree with appropriate types.
- TMXLoader also slices up tileset images and assigns the slices to the
  appropriate tiles.
- TMXProcessor can be used to further process the TMXMap objects that
  TMXLoader produces. (This is not tested, but it's very simple, so it
  should work.)
- Everything is fully documented.
- Old TMX processing classes removed.