be import-friendly: args are handled from main()
a few refinements and corner-case bugfixes
change coordinate system

- (0,0) is bottom left corner or screen
- ball.position is ball center
- positive velocity is up
add elasticity to balls
make elapsed time in ball.update() optional, defaults to TIMESTEP
fix physics bug: elastic bounces must restore full height
set colorkey to get rid of the "rattlesnakes"
interlude: rain balls!
add default pygame window icon