Implement Horse API. Adds BUKKIT-4424

API has been added to interface with Horses and to modify their inventories. Horse entities will now be recognized with the type EntityType.HORSE, and will no longer be UNKNOWN.

Previously, all Tameable animals have been animals capable of sitting. However, with the introduction of Horses in 1.6, this is no longer the case. As a result, sitting functions have been moved to their own interface so that Ocelot and Wolfs may specifically extend them.

This commit fixes BUKKIT-4393.