Client side representation and functions to operate on exercise summary data. Backbone Models and collections for exercise-summary.

Summary: It is a part of a larger feature, however contains an independent (model) part of it

Test Plan:
Internal representation of the models should be the same as JSON
returned by the server

Reviewers: jace, chris

Reviewed By: chris

Differential Revision: http://phabricator.khanacademy.org/D2232