jQuery extension, add support `scrollstart` and `scrollend` events.

/**
 * @flags
 * $.isScrolled; // boolean
 *
 * @binding
 * $(window).bind('scrollstart scrollend', function (evt){
 *     if( evt.type == 'scrollstart' ){
 *         // logic
 *     }
 * });
 */
https://gist.github.com/RubaXa/5569075