屏幕不够一屏的时候，底部固定在最下面，大于一屏取消固定

function positionFooter() {
            var footer = $("#fd");
            if ((($(document.body).height() + footer.outerHeight()) < $(window).height() && footer.css("position") == "fixed") || ($(document.body).height() < $(window).height() && footer.css("position") != "fixed")) {
                footer.css({
                    position: "fixed",
                    bottom: "0",
                    width: '100%'
                });
            } else {
                footer.css({
                    position: "static"
                });
            }
        }
        function stickFooter(){
            $(window).scroll(positionFooter).load(positionFooter).resize(positionFooter);
        }
        stickFooter();