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 documentation