I18nize the transformer widget

Summary:
Added a new i18n function that handles react things nicely, and
used it to internationalize the transformer widget.

If we decide this is useful elsewhere and isn't too expensive,
we could merge it into the original $._ function.

Test Plan:
 * Visit test.html and make sure all the english strings still
   show up correctly.
 * Cross fingers that the internationalized strings work, and
   check after those translations are complete.

Reviewers: james, alex

Reviewed By: alex

Subscribers: alpert

Differential Revision: http://phabricator.khanacademy.org/D6183