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
Add some extra helpful variables
Write real hints
Add video hint exercise
Lint down to <80 character lines