Added EntityEquipEvent.

When mobs pick up items they can equip it, for example Zombies and Skeletons.
This event is cancellable, and is called before the item is picked up and equipped.

Through this event plugin developers can know the entity involved, the item being picked up and the raw slot index for the entity's inventory the item will be equipped to.

Bukkit PR: https://github.com/Bukkit/Bukkit/pull/777