- 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.

This will leave out the student that never attempt the question and
fixes the error with histogram rendering.

There might be a chance that an error got thrown after 'contentChanged'
event was fired and stop the `toggleArrow()` method to be called. This
will make sure that the navigation will still be working even some
element in the content is failed.

PROJECT_ROOT is needed for coffeescript paths

Create dev env