Allow widgets to not be graded.

Summary:
 * Adds a checkbox to the editor for each widget to allow it to be
   not graded.
 * Adds a graded prop to the widget props, next to type and options

Test Plan:
 * Add a transformer to the question area. Make sure it has a "Graded:"
   checkbox
 * Add a non-transformer to the question area. Make sure it does not
   have a "Graded:" checkbox
 * Edit src/editor.jsx:92 to allow all widgets to be ungraded
 * Add multiple widgets to both the question and answer area
 * Try various combinations of graded/ungraded
 * Make sure that graded widgets with the wrong answer cause score to
   return incorrect
 * Make sure that ungraded widgets with the wrong answer allow score to
   return correct

Reviewers: alex

Reviewed By: alex

CC: eater, alpert

Differential Revision: http://phabricator.khanacademy.org/D5526