Added django_nose and nosexcover to requirements.txt

Capa cleanup

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

import path not consistent with dev machines?)

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