Implement asteroid

[jaredscarr]

An asteroid is an object that when introduced to the game has a size, direction, and velocity. It's position must be tracked. If it is shot then it will spawn a predetermined (?) number of smaller sized asteroids.

See code in ship class for movement.

[znschaffer]

I went ahead and refactored the Ship class out to be Entity, passing in sprite assets through the constructor - This should make it easy to reuse most of the movement and velocity logic for asteroids and eventually the saucers