Update SkullMeta to use UUIDs. Fixes BUKKIT-5614

Adds new method to SkullMeta to allow skulls to use the new UUID system and fill the necessary profile data to show skins on worn skulls.
It seems you've changed what is read from SkullOwner from the username to the UUID.  The issue with this is that this metadata will never have a UUID stored in it, as CraftMetaSkull.serialize() writes profile.getName() into that field.  And even if the serialize function is modified, it won't support conversion for skulls already in existence.