* add module_from_xml param to I4xSystem
* use it to implement xmodule.get_children()
* fix a few comments here and there
* Render-time progress display for seq and vertical modules.
  - Computes fraction of subproblems done.
* Pass problem state back to js during ajax calls.
* general cleanup in capa_module.py
* add progress_changed and progress fields to json returned from each ajax handler

* Coffeescript changes to hook up sequence tracking of problem progress

* net result: sequence 'a' tags now have a progress class
* properly set css class on initial load
* fire event when progress changes after ajax calls
* also save state in 'progress' property of problems-wrapper tag
* event handler finds those tags, computes updated progress

  - 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 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.

cleaned up some cruft prints -> logging

symmath library; add options for matrices, qubit to symmath_check

perl, and use "context" for both global and local frames