Add Edition handling

Adds  Edition handling for section item requests.

For example, I can make the following request:

    import com.gu.openplatform.contentapi.Api
    Api.item.itemId("business").response.edition

Which will return an Edition case class which looks like:

    edition: {
      id: au/business
      code: au
      webTitle: Business
      webUrl: ..
      apiUrl: ..
    }

This PR is related to https://github.com/guardian/content-api/pull/189 . But note, we can release this beforehand as calling `.edition` returns an Option - so the client will just get a `None` if no edition is found.

ps. ignore the incorrect chat in my last commit. You can access `section` and `edition` information by calling `.response` first.