Setting a horse's passenger to a non-living entity will cause a
server crash when the horse ticks, we need to check that it is
a living entity before casting, and skip otherwise.
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.
