play-json basic support

Uses play-json 2.2-SNAPSHOT from the respositories hosted by Pascal Voitot (http://mandubian.com/2013/02/21/play-json-stand-alone/)

I have created a new Unmarshaller.delegate implementation that accepts a f: A => Deserialized[B]; it's located at PlayJsonUnmarshaller object inside PlayJsonSupport trait. That allowed me to return play-json validation errors as Left.

The validation errors are returned a JSON string representation, I'm still not happy with that but was better than returning a plain error string.