Add a rainbow mode to the scratchpad

Summary: http://www.reddit.com/r/Khan/comments/1vxqq1/scratchpad_improvement/

Test Plan:
 * run python -m SimpleHTTPServer 7000
 * visit http://localhost:7000/addition_1.html
 * show the scratchpad
 * draw some rainbows!

Reviewers: eater

Reviewed By: eater

CC: alpert

Differential Revision: http://phabricator.khanacademy.org/D6317