#1: Added support for replacer in json and jsonp

* `lib/response.js`: Updated `res.json` and `res.jsonp` to accept optional replaced, updated doc
* `test/res.json.js`: Added test cases for (object, replacer), (status, object, replacer) and (object, status, replacer)
* `test/res.jsonp.js`: Added test cases for (object, replacer), (status, object, replacer) and (object, status, replacer)