code: function(scraper) {
  		var length = this.promises.length,
  			last = this.promises[length - 1];
  		if (length && last && !last.scraper) {
  			last.scraper = scraper;
  			return this;
  		} else {
  			throw new ScraperError('');
  		}
  	}
docs: Associates the current route with the a scraper (promise)
  instance. Keep in mind that the done promise will not be
  available.
  
  @param  {!AbstractScraper} scraper A scraper instance to use.
  @return {!Router} This router.
  @public