* Refactor index() so that it makes sense to me and hopefully others :)
* Rename preloaded cache of student modules to student_module_cache
* Fix line length and whitespace throughout
* add docstrings and other comments
* a few behavior-preserving tweaks to the code to make it clearer.
* Separate codepaths for with-module and without-module in index view
* Remove default chapter + section, since they don't exist anyway in course.xml
- 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.
