Fix spacing of comments (pylint)
Reorganize some of calc.py into a class `ParseAugmenter`

Holds the data for a particular parse (i.e. instantiated with a math
string and eventually holds a tree and sets of variables as well).
Add docstrings
Make preview.py work with new scheme
Add docstrings to preview.py and test_preview.py
Cleanup; add unicode test
Comment fixes
Some code fixes

Change some `is` comparison to `==`. Also, modify the `render_atom` method.
Enable the new input type for NumericalResponse
Initial commit of user documentation
Convert all usage of textline to formulaequationinput going forward

Especially, the defaults in studio to use formulaequationinputs and some of
the documentation and examples.
Change the userdocs entry about problem types to talk about formulaequationinput
Various and sundry changes to comments and whitespace
Don't use 'nada'
Add a loading icon and minimum wait for <formulaequationinput>

Wait at least 300ms between AJAX requests.
After typing, show a loading icon, when the request returns, hide the icon.