Created 2 types of enemies + land, Shooting works

Shooting now works. I created 2 types of enemies - Ufo + Ground aliens.
Ufo's are spawned above a certain y coordinate and ground aliens are
only spawned at ground level.  Both of these enemies can die.

( I deleted the old files and replaced them with the new files instead
of updating the old files because I did not realize that github was
tracking this folder instead of the other one. This is why many files
are shown as deleted)