Add editions for sections

This PR complements https://github.com/guardian/content-api/pull/190 by enabling access to the list of editions we will shortly include with sections.

To access this editions information, for example, do:

    Api.sections foreach { section => println(section.editions) }

or, for an individual section:

    Api.item.itemId("business").response.section map { _.editions } getOrElse Nil

(Section is returned as an Option for item requests so we have to map over it.)