Add a Backup() method to the API client

This is a simpler change than my previous one.  It does not try to pull out the common HTTP request code.  In the interest of getting this done sooner, here's a patch with smaller scope.  It stays closer to the original patch from perrito666/voidspace.

Note: I've decided to punt on a full functional test for backup.  It is too heavy a lift for this changeset.  For now we can just rely on the CI tests that we have for backup/restore.
"duplicate" is correct, being the adjective usage of the word.  As to that syntax, I'm not a big fan.  It makes it a little harder to read without any apparent (to me) benefit.  While I haven't looked to hard, I haven't seen anything that says it's the idiomatic way.