Add looots of comments

Summary:
Add comments everywhere! Also fix some small bugs like using Style.id
instead of Style.size, and rename some variables to be more descriptive.

Fixes #22

Test Plan:
 - Make sure the huxley screenshots didn't change
 - Make sure the tests still pass

Reviewers: alpert

Reviewed By: alpert

Differential Revision: http://phabricator.khanacademy.org/D13158