Add Directory property to GetBlobs

[kzu]

When a given container has a large amount of blobs organized in subdirectories, enumerating all
with GetBlobs is impractical.

If the caller build script can determine the subdirectory to fetch, however, things might be way more
manageable.

This could be as simple as:

var dir = container.GetDirectoryReference("");
foreach (var subDir in Directory.Split('/', '\\'))
{
	dir = dir.GetDirectoryReference(subDir);
}

var blobs = dir.ListBlobs().Select(d => d.Uri.OriginalString).ToList().Dump();

(that's working code in a LINQPad query ;).

If I get some spare time, I'd like to send a PR. But putting it here so I don't forget, and maybe you get to do it before :)