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
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
change assessments API route as suggested in review K100348
Create implicit_differentiation.html
Update exercises/implicit_differentiation.html
Update year in README
Add (c)