Optionally randomize answer choices.

Summary:
Added a checkbox to the multiple-choice/radio answer type.
When checked, students will see the answer choices for that problem in a
permuted order. The permutation is determined by the current problem
number, which is used as a seed. Problem number must be passed down from
khan/webapp/javascript/exercises-package/perseus-bridge.js

Added util.js file.

Test Plan:
    python -m SimpleHTTPServer 9000
    localhost:9000/testrender.html (refresh many times)

Reviewers: alpert, eater

CC:

Differential Revision: http://phabricator.khanacademy.org/D2293