Add directory assertions.

[klaude]

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!

[whatthejeff]

Some of this functionality was suggested previously and rejected (see #179).

I appreciate the time and effort, but I'd prefer not to add yet another assertion to PHPUnit core. This is a rather large PR and there is a real cost to increasing the amount of code and documentation that needs to be maintained for new features.

I think PHPUnit does a decent job at providing a handful of simple, useful assertions that can be leveraged to build more complex, custom assertions, if necessary. At this point, though, I'd much rather see new assertions packaged as 3rd-party plugins.

[glensc]

I think the methods got implemented now (4.8.36, I think) throws me this kind of error:

 PHP Fatal error:  Cannot make static method PHPUnit_Framework_Assert::assertDirectoryExists() non static in class TempDirTest in /builds/yUZXfEGB/0/d1/libraries/fileutil/tests/TempDirTest.php on line 96

[glensc]

Seems it was added in 5.6:

• http://www.phpunit.cn/manual/5.6/en/appendixes.assertions.html#appendixes.assertions.assertDirectoryExists