Add PathfinderGoalTame for diff visibility.
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.

HorseJumpEvent, EntityDamageEvent, and EntityTameEvent are all correctly fired for horses.

This commit fixes BUKKIT-4393.