Inject script tag into body, with callback

inject-script.js

// this takes into consideration IE7+
// http://msdn.microsoft.com/en-us/library/hh180173%28v=vs.85%29.aspx
function injectjs(url) {
    injs = document.createElement('script');
    injs.src = url;
    injs.type = 'text/javascript';
    injs.async = true;
    if(injs.addEventListener) {
        injs.addEventListener('load', callback, false);
    }
    // IE7 & IE8
    else if(injs.readyState) {
        injs.onreadystatechange = callback;
    }
    document.body.appendChild(injs);
    function callback() {
        // script was injected
    }
}