Future-proofing Perseus for coming JSX changes

Summary:
See http://fb.me/react-legacyfactory.

TODO: $_

Depends on D12798.

Test Plan: Open each widget and play with it a little. The only remaining JSX warnings are from <$_>, which I'll fix up separately.

Reviewers: aria, joel

Reviewed By: aria, joel

Subscribers: alex, jessie

Differential Revision: http://phabricator.khanacademy.org/D12796