Changes to use asteroid class's own score attributes
Moves asteroid breaking into asteroid class.

Asteroid is a single class which now takes pos, vel, and a config
object (containing score, radius, and color). Currenly only objects
for the regular and small asteroids are defined.

The break() method returns an array containing new smaller asteroids
that an asteroid has broken into (if any). They start with a bit of
mercy invincibility.

Game's removeAsteroid() method calls asteroid.break() before removing
it from its array of asteroids, then adds back whatever the method
returns.