code: function () {
  
      var args = Array.prototype.slice.call(arguments),
          port = args.shift(),
          bean;
  
      if (!this._ready) {
        throw new Error('Cannot call #listen before Kona has been intialized');
      }
  
      bean = require('./utilities/bean');
  
      if (this.env === 'test') {
        delete this.port;
      } else {
        this.port = port || process.env.KONA_PORT || this.config.port;
      }
  
      args.unshift(this.port);
  
      this.server.listen.apply(this.server, args);
  
      debug('listening');
  
      /* istanbul ignore if */
      if (this.env !== 'test') {
        console.log(bean([
          'listening on ' + chalk.dim(this.port),
          'Env: ' + chalk.dim(this.env)
        ]));
      }
  
      return this.server;
    }
docs: start the http server and listen
  @return {HttpServer} koa server instance