Support for querying creation time of files and directories

We need to find the creation time of files and directories (right now we do `File.GetCreationTime(file.PathAsString)`), so I extended functionality to support it. Basically followed how LastAccessTime was implemented, with some minor changes.
One thing I would like to discuss is if `me.VerifyIsADirectory(ThrowHelper.FileNotFoundException [...]` shouldn't be changed to throw a directory not found instead? That would apply across all `IRootedDirectory`, not just `CreationTime`, though. Also an API break of sorts.