[makehuman] Initial support to load MakeHuman human models

To add such a human model, use these lines in your Builder script:

human = MakeHuman(path/to/your/mhx/model.mhx)
[...then it behaves like a regular robot. Armature control is already setup.]
[makehuman] Workaround for MakeHuman model rendering issue
[makehuman] Add IK targets to MakeHuman models for hands and feet

Currently introduces an infortunate 90deg rotation of the
model...
[makehuman] Fix positioning of IK targets
[makehuman] Improve error handling when import MakeHuman models

- Properly check that the MakeHuman importer is enabled in Blender
- Check that the user set a skeleton in the model
[makehuman] Import only the lowpoly proxy mesh
[makehuman] Auto-activate Blender's MakeHuman importer if needed
[makehuman] Turned MakeHuman into real Robot-like

This is made possible by commit c25b05e
[makehuman] Fix renaming of IK targets