http://developer.51cto.com/art/201107/277994.htm
onscroll事件
window.onscroll|| document.onscroll
var scroll = document.documentElement.scrollTop || document.body.scrollTop;
http://bigspotteddog.github.io/ScrollToFixed/
原文：http://www.cnblogs.com/daishuguang/p/3515344.html