loadScript :p

gistfile1.js

function loadScript(url, callback) {
    if(!url) {
        return;
    }
    callback = typeof callback == 'undefined' ? function(){} : callback;
    var script_elm = document.createElement('script');
    script_elm.src = url;
    script_elm.onreadystatechange = function() {
        if (this.readyState == 'complete') {
            callback.call();
        }
    };
    script_elm.onload = callback;
    document.getElementsByTagName('head')[0].appendChild(script_elm);
}