Skulls

This PR adds in Mob Skulls, Named Player Skulls, and the Skull item, as well as all of the necessary Bukkit APIs.

Most seems to be working well. Player can place skulls on the floor/walls, rotation works on the floor, and named player heads have their skins downloaded. All of this is saved and loaded properly. Easiest way to test names is with a heads plugin, such as [PlayerHeads](http://ci.md-5.net/job/PlayerHeads/).

Thanks to @Aaron1011 and @jkcclemens for their help with this.
@Wild1145: Those two URLs are logically related (they get concatentated together along with the UUID), whereas the other UUID is less related.