Moved Wrap() to Screen.
Added GameObject abstract class.
Added Reset (screen state) methods to GameObject.
Made Asteroid inherit from GameObject.
Spawn new asteroids offscreen.
Make Explosion inherit from GameObject.
Make other game classes inherit from GameObject.
Adjusted asteroid bounding cirlce.
Force explosions to be onscreen.
Increase ship's max speed and acceleration.
Updated XnaGameLib.
Clean up drawing code.
Improve collision handling.

Collisions only occur on visible portions of objects.
Spawn asteroids just offscreen.
Change Ufo's SetDirection() to SetNextDirection().
Spawn UFOs just offscreen.
Fix missles not always spawning onscreen.