只是做个记录，以后不要自己满街找代码。。。

(function (clover) {
    clover.loadScript = function loadScript(url, callback) {
        var heads = document.getElementsByTagName('head');
        if (heads.length == 0) {
            alert("page must have one head element");
        }
        var head = heads[0];
        var script = document.createElement('script');
        script.type = 'text/javascript';
        script.src = url;
        // most browsers
        script.onload = callback;
        // IE 6 & 7
        script.onreadystatechange = function () {
            if (this.readyState == 'complete') {
                callback();
            }
        }
        head.appendChild(script);
    }

})(window.clover = window.clover || {});

// sample
// clover.loadScript("http://ajax.googleapis.com/ajax/libs/jquery/1.6.2/jquery.min.js");

　　没经过优化， 其实应该cache一点东西，并且坚持当前的script是不是存在的