function to load javascripts, based on https://gist.github.com/getify/603980

load_script.js

load_script = function (url, callback) {
  var oDOC = document;
  var head = oDOC.head || oDOC.getElementsByTagName("head");

  // loading code borrowed directly from LABjs itself
  setTimeout(function () {
      if ("item" in head) { // check if ref is still a live node list
          if (!head[0]) { // append_to node not yet ready
              setTimeout(arguments.callee, 25);
              return;
          }
          head = head[0]; // reassign from live node list ref to pure node ref -- avoids nasty IE bug where changes to DOM invalidate live node lists
      }
      var scriptElem = oDOC.createElement("script"),
          scriptdone = false;
      scriptElem.onload = scriptElem.onreadystatechange = function () {
          if ((scriptElem.readyState && scriptElem.readyState !== "complete" && scriptElem.readyState !== "loaded") || scriptdone) {
              return false;
          }
          scriptElem.onload = scriptElem.onreadystatechange = null;
          scriptdone = true;
          if (typeof callback === 'function') {
            callback();
          }
      };
      scriptElem.src = url;
      head.insertBefore(scriptElem, head.firstChild);
  }, 0);

  // required: shim for FF <= 3.5 not having document.readyState
  if (oDOC.readyState == null && oDOC.addEventListener) {
      oDOC.readyState = "loading";
      var handler;
      oDOC.addEventListener("DOMContentLoaded", handler = function () {
          oDOC.removeEventListener("DOMContentLoaded", handler, false);
          oDOC.readyState = "complete";
      }, false);
  }
};