Add expect with 0 calls options

Sometimes it's nice to verify that a method has not been called and throw an error otherwise. Of course this doesn't have to be implemented with the expect method. I would think the API could look something like this:

```
gently.prohibit(myobj, 'functionname', [optional] 'error message');
```

and once gently.verify() has been called the restriction would be lifted, restoring the old function.

Thanks for a great tool!