Finalized 2nd boss, added enemies and powerups

- death cube will enter rage mode and fly fast towards the nearest player, with lasers armed
- added a slow time powerup
- changed how enemies are spawned (continuous now!)
- added new enemy types: medic and bomber II (which patrols the area and drops bombs around the player, still a work in progress)
- fixed defense multipliers affecting health boosts
- fixed Timer.setMaxTime() not working