Removes duplicate code
Moves asteroid.game assignment to Game
Decouples randomVelocity from randomAngle

random<whatever> methods moved into the Asteroids root namespace in
constants.js. New method randomVector computes a random unit vector,
method randomVelocity now calls randomVector, then applies a magnitude
to it. The getHeading method gets an angle from a vector.
(TODO: Consider starting with angle and working from there?)