Backup client

Just so I can leave some comments. :)
Is there some mandate against moving this into its own file (e.g. backup.go)?  In my mind having a separate file for each client sub-API makes more sense (separation of concerns at least at the source file level), even if go munges everything back into a single package namespace.

In this case, I suppose it doesn't matter as much for now since there is just one function for this sub-API.  However, that won't remain the case once restore lands.  Furthermore, I anticipate that the backup-related API may even grow beyond 2 methods.  And I'd also like to see at least 1 constant for the default filename template.  So if there's that chance we might move this code into its own file later, we should do it now instead to reduce the later churn.