Numerous changes

- Asteroids now have a set (randomized) velocity to avoid stuttering
- Asteroids are now cleaned up if they go out of bounds in any direction
- Background music was added (we can change it, but we can also go for a
"delivering pizza to the space jam" story)
- Asteroid initial spawning synched with music
- Particle effects for passing stars added for the illusion of speed
- Particle effect for ship thrusters added
- Health display added
- Asteroid spawning retooled to keep a more steady stream of asteroids
coming