Basic physics done

I have the basic physics of my game going now (demo: wobbly-box.zip). Tomorrow I'll probably get collision detection going. I'm thinking of doing some kind of construction game, maybe something where the player's character builds some sort of structure to reach a series of goals.

(Edit: the readme file in the zip isn't up to date. You need pygame, pycairo, and optionally Cython. On Debian-like systems: sudo apt-get install python-pygame python-cairo cython )