WorkerName is hardcoded in sfn handler

[dremonkey]

WorkerName is hardcoded to WorkerDemo at https://github.com/Skycatch/tasque-go/blob/develop/sfn_handler.go#L69 so the data sent back to the step function execution looks like this

    { type: 'ActivityStarted',
      id: 16,
      previousEventId: 15,
      activityStartedEventDetails: { workerName: 'WorkerDemo' } }

Setting the real worker name would be nice. Might be a good idea consider passing back some additional information execution params like TASK_PAYLOAD to make it more useful. Thoughts?