New zombies + bug fixes.

- Items now disappear after 6 seconds.
- Players now have invincibility frames, fixing a bug where zombies latch
  and don't do any more damage.
- Zombies no longer collide with items.
- Added four new zombie types:
~ Daylight Walker
~ Spit Zombie
~ Gourmand
~ Devil
- Added a hardcore timer/mob spawning system which has a boss every 10
  waves.