Adds the ability to look up players by UUID. Fixes BUKKIT-5071

This commit adds the ability to look up a player by their uuid by calling
Server.getPlayer(UUID).  This is necessary for plugins which use
persistent player data.  Plugins must now store the player's UUID instead
of user name, so they can keep track of user data across username changes,
which means a method for looking up players by their UUID is necessary to
compliment the name based lookup.