Missiles: first iteration

Early version of missiles.
Only one can be fired at will. Missile isn't removed when exits the
screen. Missile is replaced if fired again.
New sprite
The missile is now unique and can only be fired again if it has first disappeared
Code clean up
Missiles: second iteration

Missiles are now fired 10 times a second when space is kept press. They
are unloaded once the sprite gets out of the screen. Only one sprite is
loaded and then pointed to when each missile is created.