Saga data class now derives from ContainSagaData instead of implementing IContainSagaData interface

Fixes #243.

Tested with sagas created for both request / response and the event correlation saga by generating code for the following canvas:
![image](https://cloud.githubusercontent.com/assets/854553/2637537/7d0af3a6-be9e-11e3-88e6-4a119291f52c.png)