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.
Not a problem, state server does not run on windows, we should have a build flag if we know that this code will/should actually not run on windows.
Also, I think you could take some of the tests from utils/tar for this that actually do the untar and dont depend on external commands.