Celtic Skull for Multi Extrusion

MacGyver's [fixed & flattened Celtic Skull](https://www.thingiverse.com/thing:70717), remixed for multi-extrusion.

I spent hours in Meshmixer carefully painting and selecting separate faces, fixing up mesh errors and splitting them out into separate bodies. The .mix file is here so you can tinker with it if so desired. The .amf file will load into Slic3r ready to be printed on a multi-extruder machine.

The split parts of the whole are: main skull, Celtic pattern on top, skull pattern o