Support basic verification modes

[rowanhill]

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

[rowanhill]

I've got a bit more stuck into it and implemented noMoreInteractions, too. I'll probably look at implementing only() too when I get some more free time.

Note that the changes in the noMoreInteractions are a fair bit larger, but much of it is just refactoring, and pulling behaviour into typed objects, so it's not as big as it first looks!

Thanks, Rowan

[rowanhill]

Okay, I've added only(). I've also modified the noMoreInteractions() commit slightly, and added some tests around verification failure messages