Add support for ajax functions on a has-many relationship

Now you can setup relationships between models in Spine and fetch the related data lazily and asynchronously!

Part of a series of ideas I have for Spine, based on ~~monkey-patches~~ freedom-patches I've been amassing in my own projects.

The idea here was that if, say, an `Author` has many `Books`, you can setup the relationship on the models and then call `author.books().fetch()` to trigger an ajax request to `/author/123/books`. 

I'm basically looking for love-it/hate-it feedback. If there's some desire to see this functionality in Spine, I'll follow up with unit tests and so forth to make this a better PR.