Improve build process

Summary:
Improve the build process by combining the CSS files, automatically
selecting font files to include, and building .tar.gz and .zip files.

Test Plan:
 - Make sure tests work
 - Make sure huxley screenshots didn't change
 - Make sure zip and tar.gz files contain only files we want to distribute

Reviewers: alpert

Reviewed By: alpert

Differential Revision: http://phabricator.khanacademy.org/D13159