PDF textbooks in Studio

This branch adds a "Textbooks" tab in Studio, and allows course authors to upload multi-chapter PDF textbooks. The frontend is all done using Backbone.js, and @chrisndodge made some improvements to the backend to better handle large files. We have Jasmine unit tests for the frontend, Django unit tests for the backend, and lettuce tests for integration. This branch also makes a bunch of miscellaneous improvements to the codebase: internationalization, tests unrelated to PDF textbooks, cleaning up messy code, a new JsonResponse subclass of HttpResponse -- and I had to restrain myself from making further, unrelated improvements.

@jtauber: This pull requests includes a few new libraries: [Backbone Associations](http://github.com/dhruvaray/backbone-associations) v0.5.0, [Jasmine Stealth](http://github.com/searls/jasmine-stealth) v0.0.12, and [django-method-override](https://pypi.python.org/pypi/django-method-override/0.1.0) v0.1.0