Wither Skeleton Horses and Wither Castles

Wither skeleton horses should spawn in soul sand valleys with wither skeletons riding them. The wither skeleton rider could have either a flame bow or a spear. Also a new structure called the wither castle, which have wither knights that spawn with axes (similar to brutes) and chests with rare loot (pottery shards, discs, diamonds, chain mail armor, etc.).