AI and player improvements

- added Medics
- melee enemies now hit you as soon as you make contact, then continue to hit at intervals
- enemies only collide with others of the same type
- added support for customizable player colors
- added armor/gear that can be worn by the player
- explosions only do damage once, not several times
- added pause/unpause to Timer class
- added throwItem method to Item.java
- added support for custom item and object functionality
- removed seeds
- dropAll() in inventory class spreads items out more