Handle API-style requests

These changes allow Geddy to behave more like an API when dealing with certain content types. I added two helper methods in base_controller so that our scaffolding doesn't get too bulky.

You still need to make some minor changes to Backbone (howdy, #382) in order to get Geddy to do content negotiation correctly. See [this gist](https://gist.github.com/ben-ng/5748312) for an example.

This is missing tests as I'm not sure how to write them yet.. Testing controllers is weird.