Implement JSONObject.fromJson() with unit tests

Gson has an API method <T> fromJson(String, Class<T>), which converts a JSON doc to an instance of class T.
JSON-Java should have a similar capability.
New unit tests to thoroughly exercise the functionality should be included.

Feature is implemented, merge pending

Closing this issue due to feature is completed in #1006. If there are any questions or issues with the fix, please open a new ticket.