Wrap List editor in CommonJS-required code

Following issue #134 and pull request #364, this pull request wraps the CommonJS-built code for the `List` editor, such that it requires the necessary modules (Backbone, underscore, and jQuery), and exports the List editor object.

This isn't, in my opinion, a necessarily nice way of doing it, but it doesn't seem to break anything, and it allows for inclusion of the list editor in backend-based testing. If preferred, I would be open to a different way of getting the List editor loaded properly, but would need advise on how to go about it.