for hints, still to be done: numericalhint, optionhint
no default hint processing done yet (ie hintmode = on_request)

  - 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

  - added correctmap.py with CorrectMap class
  - messages subsumed into CorrectMap
  - response get_score called with old CorrectMap so hints based on history are possible

  - each response can now render its own xhtml
  - cleaned up LoncapaProblem.extract_html

   - 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.

Fix histogram and sequence navigation bug

looks good, works in my tests. thanks.