Abstract Inventories & Inventory Components.

refactor how inventories are constructed to provide an API.
Adds inventory components to Zombie, Pig Zombie, Skeleton.
now have a 'Quickbar' for their held item slot, and ArmorInventory for armor.
CreatureProtocol now checks to see if an entity has Inventory and attempts to send worn/held item to clients.

Signed-off-by: Nick Minkler <sleaker@gmail.com>
Merge pull request #540 from Sleaker/master

Inventory API & EntityInventories