Allow multiple selections in the radio widget

Summary:
Add an option to have more than one selection with the radio widget.
It changes the radio buttons to checkboxes and validates that all and only the
correct answers are selected. This changes the format of the radio toJSON a bit.

Test Plan:
Make a radio exercise, click the "Allow multiple selections" box,
add some choices, make sure it works.

Reviewers: alpert

Reviewed By: alpert

CC: eater, alex

Differential Revision: http://phabricator.khanacademy.org/D2364