Added Skull Blocks / Minor update to Ender Frame Portal

Okay, so this was quite a journey as I delved deeper into the code of overviewer with very little Python experience, so bear with me.

I decided to add Skulls (or Mob Heads) as my first attempt at making a major update to overviewer.  This was a fairly major addition as it turns out that most of the important information about rendering them is in the TileEntities portion of the chunk_data and not stored in the Section itself.

Anyway this commit is the changes that I made and it is functioning well for me, although there are some nit-picky adjustments that need to be made yet. See screenshot at http://i.imgur.com/q0aCI.jpg.

Any feedback is appreciated.