Refactored segment loading and integrated changes. Further integration still needs to be done.

I made the changes we discussed. Entities are defined in a second .gif file that ends in 'e'. So level 1.1 has two files '1.1.gif' and '1.1e.gif'. I created a new file called 'entities' that maps the colors in the entity gifs to entity ids. Tile tangibility is now defined in the 'tiles' file.

The tilemap that GameWorld returns now contains tuples of the form (string tile_id, bool tangible).