- Added hints + hintmethod
  - hintgroup compatible with loncapa spec
  - also does hintfn for custom hints (can do answer history)
  - GenericResponse -> LoncapaResponse
  - moved response type tags into responsetype classes
  - capa_problem should use __future__ division
  - hints stored in CorrectMap, copied to 'feedback' in SimpleInput for display

   - responsetype used to be instantiated multiple times(!) in capa_problem
     now it is instantiated once, and stored in self.responders
   - responsetypes.GenericResponse restructured; each superclass
     show now provide setup_response (and not __init__), and may
     provide get_max_score(); general __init__ provided to
     clean up superclasses.

--HG--
rename : courseware/calc.py => courseware/capa/calc.py