Rigify: significant upgrade to arm and leg rigs, and misc changes/bugfixes.

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.