Return a Users UUID as a UUID object whilst keeping support for returnin...

...g as a String

I added a new method (getUniqueId()) to the ProxiedPlayer interface to return a users UUID as a UUID object (before it was returned as a String). I had to add this as a new method with a slightly different name as changing the return type of getUUID would break any plugins using it. I instead deprecated it ;)

Methods added to the Util class are the same ones used by Minecraft [here](https://github.com/Bukkit/mc-dev/blob/master/net/minecraft/server/UtilUUID.java)

Maybe the getUUID method can be removed once everyone has switched over.