Add API for damage modifiers in EntityDamageEvent. Adds BUKKIT-347, BUKKIT-4104

This commit adds API for the enchantment, armor, potion and other modifications to
damage done to an entity.  These damage modifiers are each editable via a getter
and a setter.  This addition allows for more accurate modification and monitoring
of damage done to/by an entity, as it displays the final damage done as well.