Don't create `exports` variable in a browser

Summary:
The `var exports;` was creating an exports variable globally, which we don't want to do. Refactor code slightly so that only StructuredJS is created, not the others.

(I didn't know who to send this to, sorry!)

Test Plan: Tested in-browser and with command line tests.

Reviewers: leif, sophia, pamela

Reviewed By: leif

CC: ben

Differential Revision: http://phabricator.khanacademy.org/D4108