Add Horse API. Adds BUKKIT-4424

API has been added to interface with Horses and to modify their inventories. A new event, HorseJumpEvent, has been added to be fired whenever a horse jumps.

This commit fixes BUKKIT-4393.
Minecraft spec has changed and we're required to follow. We now build with Java 6.
Merge branch 'master' of https://github.com/Bukkit/Bukkit into master-spigot