Add NSAttributedString support to haveCountOf matcher

This adds `NSAttributedString` as a compatible type for the `haveCountOf` matcher. I have found myself wanting this, and I think it makes sense given its close relationship to `NSString`.

I thought about checking if the `actual` object responds to `-length`, but I worried that it would too aggressively apply count-like semantics to all `-length` methods. In the end, checking for the `NSAttributedString` class was the quickest and easiest route,

Thanks for such a great project!