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.
Yep, sorry I should have seen that.  

I don't have a strong feeling on proxy vs not proxy but proxy didn't resonate with me. It's a which has Write as it's main interface so calling it a ....Writer seems best to me. 

Your call.