Modified ship controls to be more sensible. Recolored asteroids. ALSO,
IMPORTANT: All update(int) methods changed to update(float). At first
the parameter was the number of milliseconds since the last frame, but
now it is the number of seconds since the last frame as a float. This
makes it easier to step things like physics functions independently of
the FPS (e.g. to move at 1m/s, add 1*delta to the position every frame)