Feature/master/migrate api to siren

These patches change the output of all endpoints to JSON [Siren](https://github.com/kevinswiber/siren), and adds helpers to /app.rb to make it easier to add Siren actions (form submission endpoints) to collection endpoints. This isn't quite ready to be committed yet, since the CLI doesn't support object creation, but I'd like some feedback if possible.

Also, I realize that PuppetConf is a few days away, so the (backwards-compatible) overhaul of the API in this PR might not be the best thing for Razor right now.