load script
function loadScript(url, onLoaded) {
var loaded = false;
var scriptTag = document.createElement('script');
scriptTag.src = url;
scriptTag.onload = scriptTag.onreadystatechange = function(){
if(!loaded && onLoaded) {
loaded = true;
onLoaded();
scriptTag.onload = scriptTag.onreadystatechange = null;
}
}
document.head.appendChild(scriptTag);
}