I touched up some of the introduciton screens.

I removed some debugging code (e.g., System.out.println() etc...)

I fixed a bug in the Asteroids class that prevented the UFOs from being removed after they have moved across the screen and were not hit by the player.

Most importaantly, I moved all of the display logic into a screen manager.
Added ship exploding into pieces when player dies.
i tweaked the code a bit that causes the ship to break into pieces when the player dies.