Introduce BodyParsers.parse.json(Reads[A])

This is a BodyParser that not only validates the content type as Json and parses the body to a JsValue, but then goes on the validate the JsValue according to the implicit Reads[A].
While BodyParser.validate can achieve something similar, the function type (f: A => Either[SimpleResult, B]) means that one cannot use Futures and thus invoke GlobalSettings.onBadRequest.
Merge pull request #1860 from dwhjames/topic/readJsonBody

Introduce BodyParsers.parse.json(Reads[A])