Added ThornsEvent.

Added 2 new events.
*ThornsEntityDamageEvent - Called before Thorns damages the damaging entity. Cancelling will not cause the entity any damage.
*ThornsArmorDamageEvent - Called before Thorns damages armor. Cancelling will not cause the armor to be damaged.

This will be useful for armor that does not use the standard damage system, or armor that does not take damage when applied with thorns.

Example: http://pastebin.com/6divcazh