From the creator of Trainguy's Iron Golem rig, I present Trainguy's Skeleton Rig! Features include 2 segments for the arms and legs, an opening jaw, 3D skeleton, a bow, arrow, and 3D quiver. The bow can be pulled back with a shape key, but it's not perfect (It's good enough, it's just the small details). I got the idea of the jaw from one of MineworksAnimations' videos. I spent a lot of time on this, yet there's still more that I can add.
If you want a wither skeleton, just get the wither skeleton texture in the minecraft.jar file. If you replace the defualt skeleton texture with it, it should show up perfectly. If you want the correct size for it, just select the main bone and press [S] and type [1.333].
Version 1.2: Updated rig used in TheDuckCow's MCPrep. Get it here: bit.ly/MCprep. Changes include small enhancements to the bone shapes/names. Credit goes to @TheDuckCow for changes in v. 1.2