issue 67 solved. added 3 new issues about missing api picture check and get_api_picture_url_<provider> methods

 67) check just_posted variable in get_api_picture_url_<provider> methods
     just_posted = true if called from generic_post_on_wall, just_posted = false if called from missing_api_picture_urls
     different error message after post on wall and after missing_api_picture_urls check.
     see get_api_picture_url_facebook. facebook has a special read post on wall permission (grant read permission link)