very littered first pass at persistent holes.

Hole persistence works,  however holes need to also be rendered with any transforms above them, and this doesn't yet work
Added inefficient but functional hole behavior.  Calling obj.set_hole( True) for any SolidPython object will cause it to be subtracted from the scene after everything else.
Made hole transforms more efficient
-- Changed add() so that objects are always copied when re-used in Python code.  This was needed so that we could re-use hole() objects without having to copy() them first
-- Added hole_example.py
Added Hole to README.md; Improved headline hierarchy
readme
Merge branch 'sp_persistent_holes_20130219'

* sp_persistent_holes_20130219:
  readme
  Added Hole to README.md; Improved headline hierarchy
  -- Changed add() so that objects are always copied when re-used in Python code.  This was needed so that we could re-use hole() objects without having to copy() them first -- Added hole_example.py
  Made hole transforms more efficient
  Added inefficient but functional hole behavior.  Calling obj.set_hole( True) for any SolidPython object will cause it to be subtracted from the scene after everything else.
  very littered first pass at persistent holes.