Add an ArchiveData wrapper type.
Expose the Open method.
Fix up some doc comments.
Drop a superfluous assignment.
NewArchiveFile -> NewArchiveData (and add data parameter).
af -> ad.
Pull ArchiveData out of the archive package.
Add a test for legacy version.
Adjust to upstream changes.
Merge pull request #1053 from ericsnowcurrently/backups-archive-version

Add a backup archive data wrapper type.