Animation

Moved rendering of enemies to super class, implemented animation. May
later make AnimationState a private enum.
Merge branch 'feature-animate' into develop
Enemy bullets

Enemies capable of shooting bullets. Right now, they just shoot based
on a low probability. Bullets to not destroy fighter.
AppConstants comments
Fighter destruction

Fighter can now be destroyed by enemy bullets. At this time, the game
loop halts.
Merge branch 'feature-enemybullets' into develop