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"
```

Haven't actually tried yet, here's an example I found describing how to do it though: https://gist.github.com/kenoir/4635997b94942be146ef