SPACE INVADERS!

This is a simple "space invaders" type game in which you move your "gun"
back and forth to shoot at invaders who are decending from above. Above
the gun are "shields" which protect you from alien fire, but disintegrate
a little with each hit.

Both you and the invaders can have up to 5 concurrent shots visible on the
screen. Your gun holds 5 shots, and reloads one shot every 10 "ticks" of
the game. (Each tick is one movement of the invaders).

Use the LEFT and RIGHT arrow keys to move your gun. SPACE fires. ESCAPE
quits the game.
