- 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.

Refactor and test Dynamath Code (tested, works; one more changeset coming, to add msg back into textinput_dynamath)

fix dogfood wrt lms / xmodule reorg; fix textinput_dynamath call

Removing references to django.conf.settings from capa/responsetypes.py

Problem testing utility, reducing courseware/capa dependencies on Django

fix breakage in problem checker caused by LoncapaProblem taking a file object in its constructor instead of a filename

customresponses can't tell us what their answer is (there can be a
wide range of answers -- it's verified by a small program). So here
we simply ignore those fields we can't generate answers for when
doing tests against problems.