自动切换无法加载的静态资源，预防 CDN 挂掉后网站故障

<script type="text/javascript" src="//apps.bdimg.com/libs/jquery/2.1.4/jquery.min.js"></script>
<script>
(function(){
 window.jQuery || document.write('<script src="/Public/static/js/jquery-2.1.4.min.js"><\/script>');
})();
</script>

<script src="//cdn.staticfile.org/jquery/1.8.3/jquery.min.js"></script>
<script>
   if (!window.jQuery) {
    var script = document.createElement('script');
    script.src = "/js/jquery.min.js";
    document.body.appendChild(script);
}
</script>