example-widget: use JsonifyProps

Test Plan:
 * make an example-widget
 * try grading an empty text box
 * try grading a wrong answer
 * try grading a correct answer
 * try the toJSON button

Reviewers: brianmerlob

Reviewed By: brianmerlob

Subscribers: alpert

Differential Revision: http://phabricator.khanacademy.org/D7470