Missed committing this earlier; update to our wither skeleton detection code to use newish type check provided by Bukkit, fixing the ClassNotFoundException error given for CraftSkeleton in new version of CraftBukkit

Also corresponds to release 1.5.2
Messages containing %killer_item will now give the custom item name for items which have one (via an anvil or whatever), otherwise defaulting to the standard item name
Release 1.5.3