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

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

added a posix compliant check to make sure you are using bash to run the script, added option to use --system-site-packages for virtualenv, updated the ending instructions on how to start the server

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.