Equivalent Fraction 3 - new exercise

Summary:
- Two problems as described in the "Smarter Balanced task" spreadsheet

- I suspect there is some cruft, but I couldn't find a nicer way to handle
the jQuery UI checkbox/button without the function as they need matching
id's on the label and input.

Test Plan: locally, Chrome/OS X

Reviewers: eater

CC: alpert

Differential Revision: http://phabricator.khanacademy.org/D2130
Bug fix for button drawing.
Merge branch 'equivalent-fractions-3' of https://github.com/davemajor/khan-exercises into equivalent-fractions-3