模拟scrolltop

[fayching]

$.fn.scrollTo = function(scrollHeight ,duration) {
            var el = this;
            var startPosition = el.scrollTop();
            var delta = scrollHeight  - startPosition;
            var startTime = Date.now();
           function scroll() {
               var fraction = Math.min(1, (Date.now() - startTime) / duration);
               el.scrollTop(delta * fraction + startPosition);
               if(fraction < 1) {
                   setTimeout(scroll, 10);
               }
           }
           scroll();
       };