Updated Json behaviour so it doesn't stringify everything

See for context:

https://groups.google.com/forum/#!topic/cascading-user/BqaQ5SOoZ3E/discussion

I tried json4s, parsing.json.JSONObject and spray-json before going back to Jerkson:
* json4s and spray-json didn't seem to support JSONifying a `Map[String, AnyRef]` out of the box. Weird
* Jerkson and parsing.json.JSONObject unfortunately convert a Scala value of 42 to 42.0 in JSON. Not sure why. I have neatly sidestepped this in my updated test

Anyway, I think this implementation is better than what went before! Hope the Etsy crew don't mind it.