Added a json_object_new_double_s() convenience function to allow an exact...

Added a json_object_new_double_s() convenience function to allow an exact string representation of a double to be specified when creating the object and use it in json_tokener_parse_ex() so a re-serialized object more exactly matches the input.
Add json_object_free_userdata() and json_object_userdata_to_json_string() too.