Improved FireFly and Enemy's method structure
-FEATURE: Improved FireFly's movement when not in orbit.
-FEATURE: Added collision to FireFly.
-FEATURE: Added so FireFlies copies the rotation of the "soul".
-FEATURE: Added removal of a FireFly's captured soul.
-FEATURE: Added the possibility for Conversion effects to "grow".'
-FEATURE: Added soul releasing to FireFly (untested).
-OTHER: Added content to todo.txt.
-CODE: Added a removeEnemy()-method to Enemy.
-CODE: Removed a couple of TODO's and XXX's.
-CODE: Migrated Enemy's collision code to a collisionCheck()-method.
-CODE: Added a getRadius()-method to Enemy.
-CODE: Changed Enemy's collision(Player) to collision(GameObject).
-CODE: Added the Killable interface and added it to Enemy and Player.
-FIX: Fixed the resetting of the SpawnSystem on game restart.
-BUG: Enemy's collision(Player)-method was never called in
collisionCheck().