Animation with spritesheets, and bug fixes!

- .animation is now a method of all doodads, and player objects. This
method sets up the spritesheet for animation based one some arguments
(fps, number of frames, height & width).
- Fixed a bug with the camera being attached to the core.player object,
instead of the actual player. This caused som lag, which is gone now!
- Fixed a bug where the resize event would try to use the old
engine.set_camera function.