http://blog.csdn.net/chenxi1025/article/details/52222327
https://www.cnblogs.com/ajianbeyourself/p/5815689.html
jQuery.fn.center = function () {
this.css("top", ( $("body").height() - this.outerHeight() ) / 2+$(window).scrollTop() + "px");
return this;
}
$(".hint").center();