gm: initial documentation
gm: lots
gm: lots (again)
gm: lots more
integrate gm into Game, plus other stuff
cleanup Game stuff

* remove overlay and fade stuff
* store worlds as just the world, not a dict of attributes, and clean up
  world methods accordingly
start_world, switch_world can take World instance
easier access to current FPS
World.select
some Graphic setters
gm.Image