Add MHF (Mojang Head Format) heads to the list and

add magma cube and wither heads, along with other backup heads.

I collected the MHF heads from Reddit, where Marc_IRL posted a bunch of mob heads. (http://www.reddit.com/r/Minecraft/comments/1m0itx/twitter_marc_irl_mapmakers_would_it_be_useful)

I collected the magma cube, wither and backup heads from http://imgur.com/a/iUfDf

This would be a nice addition to add.