Add Play support 

In order to provide feature-switching in the [Workflow](https://github.com/guardian/workflow) app written using the Play framework, this PR implements Play controller with Actions matching endpoints that exist in the current Scalatra implementation.

This PR implements actions only for:

- GET /api/features
- GET /api/features/switches
- GET /api/features/switches/:key
- GET /api/features/switches/:key/enabled

Further PRs will follow to add remaining endpoints.

To run tests:

```sh
sbt "project play" "test"
```

Currently it returns `true` (just the 4 characters) but that's not valid JSON (and it looks like the mime-type is wrong too). I'd go with your suggestion above, which also matches the `argo` spec (which doesn't really exist in writing atm, it's a "known bug") used for the rest of the API!

Ideally could return `application/vnd.argo+json` as media-type for all these endpoints.