Most of the way done with implementing support for export to S3 buckets
with versioning enabled. This will make the files from the most recent
`git annex export` be visible to users browsing the bucket, while letting
git-annex download any of the content from previous exports too.

Still need to test it. And, deletion of old content from such a bucket is
not supported, and my initial thoughts are that it might not be possible
in a multi-writer situation. I need to think about it more.

This work is supported by the NSF-funded DataLad project.
