NOTE: I didn't get around to fixing any bugs from the previous play test, so those bugs are still there. I'll get to them in the next few days.

Added:
- 2 new Bosses. 1 for mission 3, and one for mission 6. Mission 3 now randomly selects the boss, so replay mission 3 to see different bosses. Mission 6 only has 1 boss for now.

Fixes:
- fixed rare items not spawning in Mission 4
- reduce physics force when enemy is killed