Player skulls with SkullOwner do not use UUIDs; can break after name-changes

Player skulls with the SkullOwner tag do not store their UUID. As a result, if a user changes their name, all existing copies of their skull will be broken. It's hard to say skulls aren't intended to work in the long term (as items and building blocks, there are plenty of reasons to keep them around for a long period of time). There are also no particular benefits to using names of nonexistent players as SkullOwners, although the system should probably accept both names and UUIDs when a custom skull is first spawned.

Steps to Reproduce:

1. Spawn a custom player skull, using a command such as

   give @p skull 1 3 {SkullOwner:"WolfieMario"}

2. Observe that, when placed, it displays the skin of the named player.

3. Save and quit

4. Using an NBT editor, examine the skull as it is saved in your inventory. The SkullOwner remains a String tag of the player's display name, meaning it will be broken if this player changes their name, and may even become a different player's skull if somebody else takes the name.

This will cause issues when name-changing is available to the public. Until then, the bug won't have any in-game effects, but 1.8 will already be released before name-changing is possible, and by then it would be too late to fix this (barring a 1.8.1, of course).