Add onCall(n) and friends to SinonStub definition

These were added in Sinon 1.8, and the definition claims to be Sinon
1.8.1

From documentation:
"As of Sinon version 1.8, you can use the onCall method to make a stub
respond differently on consecutive calls."

http://sinonjs.org/docs/
verified 

![image](https://cloud.githubusercontent.com/assets/874898/3347734/3cc7b724-f8f6-11e3-9b3e-a733dfe2a97a.png)