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.