exercise summary backend

Summary:
Endpoints for the exercise summary UI. Will have to be expanded to deal with problem types

Backend for exercise_summary. Simple fetches from mongodb

making layout consisten - 1 minimal change

Test Plan: Would be great to find one. Open to suggestions.

Reviewers: jace

Reviewed By: jace

Differential Revision: http://phabricator.khanacademy.org/D2161