Finish callback step with external callflow and retry option

Callback step can use external  call flow. It is a good design to isolate callback to another call flow  ![screen shot 2014-10-17 at 2 36 51 pm](https://cloud.githubusercontent.com/assets/4484749/4675752/7332455e-55d0-11e4-9b2e-ffb6c39d00b4.png)