code: function(callback) {
  		this.promises.push(function then(done, utils) {
  			done(null, callback(utils.lastReturn, utils));
  		});
  		return this;
  	}
docs: Sets a generic promise.
  
  @param  {!function()} callback Callback.
  @return {!ScraperPromise} This object, so that new promises can
  be made.
  @public