AT-AT refactored

Kolo33's AT-AT model has some nice design detail but is hard to print, and the pose is somewhat arbitrary. This is an improved version.

I fixed problems related to too-thin geometry, strange internal faces, areas that didn't join properly and so on. I also divided the model into parts based on the natural joints and added allowances so they can rotate. This allows the model to be posed, although the final pose will need to be glued to be able to hold its own weight.

Completed model stands 30 cm high. Based on: http://www.thingiverse.com/thing:215574