code: function(callback) {
  		this.promises.push(function async(done, utils) {
  			callback(utils.lastReturn, done, utils);
  		});
  		return this;
  	}
docs: Stops the promise chain and resumes it after a callback
  function.
  
  @param  {!function(!function, !Object)} callback Callback.
  @return {!ScraperPromise} This object, so that new promises can
  be made.
  @public