Day #4 and #5

Finally, there is enemies. Still it is getting stupid. I am finding joy in coding game that is not fun...

I need to:
- Add more enemies
- Add some subtle animation
- Begin/End
- Tweak a lot of stuff until it is fun

Last point seams impossible but who knows.

By the way, I am using pymunk just for collision detection and pymunk.vec2d.Vec2d class because it rocks. I like this my idea, because it works even if You change position by hands and not with gravity or `add_force` or `add_impulse`.