Implemented large chunks of the overloader module. Sprites are now
loaded from a level descriptor and stored efficiently in the level.
Layers now automatically get their SpriteBatches if sprites are added
and can efficiently be iterated over via ipairs. This is demonstrated in
love.draw() in main.lua where some dummy sprites from the tutorial level
are rendered (with more than 600 fps btw).
Overloader now also has some helper functions to quickly find the right
layer or SpriteBatch. Additionally documentation has been written
regarding sprites and their use in FOSSBRAWL.