Widgets Exposed! See the renderer's revealing refactor

Summary:
Expose methods on Renderer and ItemRenderer for
returning a map of widget ids to scores.

Test Plan:
 * Make a renderer
 * add an input-number
 * try various types of grading
 * run `itemRenderer.scoreWidgets()["input-number 1"] in the console
 * `make test`

Reviewers: alex

Reviewed By: alex

Subscribers: johnsullivan, dylan

Differential Revision: http://phabricator.khanacademy.org/D13763