code: function(callback) {
  		if (this.creating && callback) {
  			this.waiting.push(callback);
  		} else {
  			var that = this;
  			this.creating = true;
  			phantom.create(this.flags, this.options, function(ph) {
  				that.instance = ph;
  				that.creating = false;
  				that.waiting.forEach(function(callback) {
  					callback(ph);
  				});
  				that.waiting = [];
  			});
  		}
  	}
docs: Creates PhantomJS instance if needed be, and when it's done
  triggers all the callbacks.
  
  @param  {!function(?)} callback Function to be called when the
  instance is created, if a phantom instance is waiting to be
  created the callback will be added to a waiting list.
  @private