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.
The "/" trick is just to support passing in a dirname without having to bloat the function signature.  It's not like we'll get a real filename with a trailing slash (posix doesn't allow that).  Also, what do you mean by using filepath.Join?  We've already verified that the name ends with /, so using filepath.Join wouldn't do much, would it?