Add ke (khan-exercises) submodule.

Test Plan: load the test.html
Get unit tests working with JSX.

Test Plan: ran qunit tests.
Begin work on dropdown.
Add a dropdown widget (for things like <, =, >)

Summary:
Add a dropdown widget.

also:
* Add ke (khan-exercises) submodule.
* Get unit tests working with JSX.

Test Plan:
Try the dropdown in both test.html to verify editting and
toJSON. Also try it in testrender.html to verify scoring.

Then try it locally with a built perseus.js in the webapp.

Reviewers: alpert, eater

Differential Revision: http://phabricator.khanacademy.org/D2563