code: function (heartbeat, cb) {
          var self = this;
          this.swfClient.recordActivityTaskHeartbeat({
              taskToken: this.config.taskToken,
              details: stringify(heartbeat)
          }, function (err) {
              if (cb) {
                  cb(err);
              }
  
          });
      }
docs: Sends a heartbeat to AWS. Needed for long run activity
  @param {Mixed} heartbeat - Details of the heartbeat (will get stringified in JSON if not a string)
  @param {Function} [cb] - callback