waitForScript

/**
* Only returns true when the external script has been
* loaded in to the DOM.  It uses arguments.callee.caller
* to work out which function is the callback.
*
* @param url {String} URL of external script
* @param obj {String} The name of a function or variable
* within the external script to test for.
* @license: Creative Commons License - 
* ShareAlike http://creativecommons.org/licenses/by-sa/3.0/
* @author Remy Sharp / leftlogic.com
*/
function waitingForScript(url, obj) {
  // doesn't work in Opera
  var callback = arguments.callee.caller;
  var args = arguments.callee.caller.arguments;
  var s, ok, timer, doc = document;
 
  // if the object/function doesn't exist and we've not tried to load it
  // then pull it in and fire the calling function once complete
  if ((typeof window[obj] == 'undefined') && !window['loading' + obj]) {
    window['loading' + obj] = true;
 
    if (!doc.getElementById('_' + obj)) {
      s = doc.createElement('script');
      s.src = url;
      s.id = '_' + obj;
      doc.body.appendChild(s);
    }
 
    timer = setInterval(function () {
      ok = false;
      try { 
        ok = (typeof window[obj] != 'undefined');
      } catch (e) {}
 
      if (ok) {
        clearInterval(timer);
        callback.apply(this);
      }
    }, 10);
 
    // we're loading in the script now, so we're currently waiting
    return true;
  } else if (typeof window[obj] == 'undefined') {
    // object not defined yet, so we're still waiting
    return true;
  } else {
    // it's already loaded
    return false;
  }
}