Add human-readable date output option

In order to enable the changes agreed to in https://github.com/cloudfoundry/cloud_controller_ng/issues/181, adds a readable_dates option which formats dates in a way that's still machine readable, but is also human-friendly. I considered changing the default, but in case anyone is relying on the previous date format it seemed safer to expose as a config option which defaults to the old behaviour. 

I'm also open to the idea of adding a ReadableJson codec or similar rather than modifying the existing Json codec, but that seemed like more complication than this change warranted.