code: function (config, cb) {
          var w = new WorkflowExecution(this.swfClient, this.config);
          w.start(config, cb);
          return w;
      }
docs: Creates a new Workflow instance and start it
  @param {Object} config
  @param {Function} [cb] - called once the workflow execution started
  @returns {WorkflowExecution} workflowExecution - The new instance of the workflow execution