Don't load all levels at once. Load them as needed

[laplante-sean]

RIght now the init() method in Level reads the map and builds the entity lists. Break this code out into a load() method and only call it when the level is the current level.

Maybe:

1. Maybe create a loading page if loading takes a while but it probably won't take long.
2. Maybe go through and clean everything up on unload. Or just rely on python garbage collection.