enemies can get destroyed, preparation for #4
take rotation into account when checking for collisions
fewer vertices
added bodyCount and Health, startet #5 and #8
split systems into more files
added HUD and display death toll for #13
added some enemies
fix #8
added very important bubble bursting sound
added explosion and impact images, show explosions on impact
show explosions on destruction
spawning enemies
collision checks only for bullets vs units
improved spawning, disabled debugsystem
a few more sounds and a fixed airplane