code: function (config, cb) {
        var w = this.start(config, function (err, runId) {
          if (w)
              w.runId = runId;
          cb.call(this, err, w);
        });
      }
docs: Just like `start()`, but callback passes the workflow execution with the
  run ID with no return value
  @param {Object} config
  @param {Function} [cb] - called once the workflow execution started
  @returns {WorkflowExecution} workflowExecution - The new instance of the workflow execution