Significant Progress for Combat System

Characters have health (no display)
Damage reported via System.err for now
Weapon hierarchy slightly refactored, there is now an Equippable
abstract superclass. Implementing classes: Weapon, Shield
IAttackable interface added for things that can be attacked.
Implemented classes: Character, Shield

Fleshed out StatusEffect Hierarchy and classes.  They now follow the
prototype pattern, where the original is not used, and a copy is made
when the effect is applied.  This is used to grant limited immunity to
status effects.

Currently, main melee weapon has poison attached.

When a character is struck, granted 1s immunity to further strikes,
flashes. No knockback yet.

Shield can block melee/projectile damage if projectile/melee strikes
front of character with shield equipped.

Some refactoring of Character.

Maybe other stuff too.