Give access to what arguments were passed with in the signal

Thanks for the great plugin!

Would be awesome if when using qtbot.waitSignal, you could access any arguments that were sent in the signal e.g.

``` python

with qtbot.waitSignal(worker.success) as blocker:
    pass
assert blocker.args == 'hello, world!'
```

What are your thoughts?

I'll take a stab at it if you think it's a good feature, thanks
