Allow root path of "/"

[bizurkur]

Copied from #64:

I propose being able to store files directly into the vfs without first having to create an arbitrarily named directory to store everything in since I believe that is counter-intuitive for new users. I propose doing this by calling vfsStream::setup('/');, or better still, make it the default behaviour when calling vfsStream::setup();.

Changing the default would cause a BC break and force everyone who accesses the current default "root" path directly to update their tests.