Start integer sum exercise
Working integer sum exercise
Implemented some suggestions
Improved final hint
Add case-insensitive flag to regex answer type
Add partial fraction expansion exercise
Change wording
Add partial fraction expansion exercise

Summary:
Added a partial fraction expansion exercise based on Sal's partial
fraction expansion 1 video.

Reviewers: eater, alpert

CC: stephanie

Differential Revision: http://phabricator.khanacademy.org/D1054
Fix spacing, expr, silly problems
Lint down to <80 character lines
First pass at creating a new extract-strings.py script for our i18n efforts.

Summary:
This is a simple script that's able to extract strings from the Khan Exercises .html files and generate a .po file.

I haven't evaluated how accurate the results are yet - and I've found at least one major issue which I'll resolve tomorrow, namely that elements inside of banned elements (like `<code>`) are still being picked up.

Test Plan: Run `python build/extract-strings.py`. You should end up with a file named `exercises.po` containing a number of translatable strings.

Reviewers: csilvers

Maniphest Tasks: T340

Differential Revision: http://phabricator.khanacademy.org/D1058
connect assessments engine (selects next exercise) to frontend via new API routes
Add some extra helpful variables
Write real hints
Add video hint exercise
Lint down to <80 character lines
Rewrite of extract-strings to be easier to use and integrate feedback from Craig and Alpert.
Fix some linting issues.
Fix line spacing for lint.
Add in a new extract_strings_test.py file for testing string extraction. Fix a new bugs in extract_string.py, as well.