Previously, for a textbook with only one chapter, we allowed that chapter to
not have a title -- under the assumption that it would be shown as a monolithic
textbook, instead of a chaptered textbook. However, the UI doesn't support that
idea, the code was getting a bit messy, and there was no real benefit to the idea
of not having to specify a chapter title. This commit removes that special case,
and ensures that all textbook chapters must have a name and an asset path.

They are the defaults now

But only slightly. Sorry Christina.

Replacing the get_request_method function everywhere

Previously, when a model was deleted, the entire collection would save itself
to the server again. Now, we just call model.destroy(), which doesn't affect
other models.

Created a few RESTful API endpoints, which required creating and assigning
arbitrary IDs to PDF textbooks. Changed the Backbone views to save individual
models, instead of saving a whole collection.

Based on http://djangosnippets.org/snippets/154/

minShown = 1250, closeIcon = false

Previously, the code would dynamically add a chapter on render() if the textbook
had no chapters, and would remove all empty chapters when the edit view was closed.
Now, the render method doesn't modify the model at all anymore (yay!) and when the
edit view is closed, remove all empty chapters but be sure to leave at least one
left.