Added JSON rendering method.
Added JSON rendering method.

* Split (_render) into (_render_html) and (_render_json). By default (_render)
  calls (_render_html)
* Added (_render_object_as_json) method. It's used by the (_render_json)
  method and also when creating a JSON response that doesn't use the
  member or collection of the controller--i.e., when you want
  to JSONify an arbitrary object.

wylee/restler