code: function create (path, fn) {
    if (scripts[path]) return fn();
  
    var el = document.createElement('script');
    var loaded = false;
  
    // debug: loading "%s", path
    el.onload = el.onreadystatechange = function () {
      if (loaded || scripts[path]) return;
      var rs = el.readyState;
      if (!rs || 'loaded' == rs || 'complete' == rs) {
        // debug: loaded "%s", path
        el.onload = el.onreadystatechange = null;
        loaded = true;
        scripts[path] = true;
        fn();
      }
    };
  
    el.async = 1;
    el.src = path;
  
    var head = document.getElementsByTagName('head')[0];
    head.insertBefore(el, head.firstChild);
  }
docs: Injects a script. Keeps tracked of injected ones.
  
  @param {String} path
  @param {Function} callback
  @api private