Added auto-stretch property to ik limbs.  Also misc bug fixes.

Bug fixes:
- "Stretch FK" and "Stretch IK" were confusing for animators.
  Changed to "Length FK" and "Length IK".
- Auto-stretched IK limbs didn't match perfectly with FK chains
  even when not stretching.  Fixed.
- Several bugs in the IK/FK snapping code that made it fail
  in various circumstances.  Fixed.