Add damage modifier API 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 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.