Support basic verification modes

I've added some basic verification mode functionality, to allow matching exact, max and min number of calls.

I haven't implemented `only()` or `noMoreInteractions()`, as they require a bit more effort (and, I suspect, aren't as frequently used). I also haven't done `calls()`, since that's to do with ordered verification, which I haven't tackled here either.

I think it's all fairly straightforward, but let me know if you have any questions or if there's anything you'd like me to change.

Thanks,
Rowan