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.
With only a small change, hashingWriter(Proxy) could be used here and make this function tiny. io.Copy needs a io.Writer and hasingWriter satisfies that interface right?

hashingWriter just needs a method that returns the hash without base64 encoding. Maybe .Hash() should return the output of hasher.Sum() and another method should return the base64 encoded one?