Support iterating over members of a JsonObject

* Introduce inner type JsonObject.Member to represent a single member
  (i.e. a pair of name and value) of a JSON object.
* Provide an iterator() method that returns an iterator over the
  members of the object.
* Let JsonObject implement Iterable<Member> to allow for using it in
  enhanced for loops:

    for( Member member : jsonObject ) {
      String name = member.getName();
      JsonValue value = member.getValue();
      ...
    }
Improve test coverage