Arm/leg rig upgrades:
- Arms and legs no longer scale with their parents, which was
problematic when e.g. the torso of a character did
squash-and-stretch causing the arms/legs to distort and shear.
- Squash-and-stretch for both FK and IK rigs.
- Rubber hose controls.
Misc changes/bugfixes:
- Rigify now locks all pose transforms for non-control bones
automatically.
- The README file now correctly reflects the new rig-type API.
- Scrubbed the code for unused variables and imports.
- PEP8 cleanups.
This makes it much easier for e.g. someone to branch Rigify for
custom purposes, since there won't be weird name conflicts.
Also changed from using __import__() for dynamic imports to using
importlib.import_module(). This simplifies the code and should
be more robust.
Finally, misc pep8 cleanups.
