Add FileHelper::isEmptyDirectory() and improve FileHelper::unlink()

[vjik]

Q	A
Is bugfix?	✔️
New feature?	✔️
Breaks BC?	❌
Fixed issues	#21

[samdark]

Here's a simpler fix: #25

[samdark]

I don't think it was bad in a single method.

[samdark]

Let's do the following:

1. Merge windowsUnlink() with unlink(). There's no need to have 2 methods and original one is clear enough.
2. Add phpdoc for isEmptyDirectory().

[vjik]

Let's do the following:

1. Merge windowsUnlink() with unlink(). There's no need to have 2 methods and original one is clear enough.
2. Add phpdoc for isEmptyDirectory().

Done