Add directory assertions.

Add the following assertions:
* `assertDirectoryExists()`
* `assertDirectoryNotExists()`
* `assertDirectoryCount()`
* `assertDirectoryEmpty()`
* `assertDirectoryNotEmpty()`

I was working with directory processing for a project at work, and figured these assertions may come in handy. I think I've followed the conventions present in the existing code, though I've left `@since` docblock tags out since I don't know which version it'd go into, provided it's merged. Thanks!