Add the client side of the backup API.

This builds on the work of Michael Foord.  I've reorganized the code a little, added tests and factored out the raw RPC code from the backup client code.  (The raw RPC can later be re-used by the other 2 client methods that currently duplicate the code.)
probably shouldn't be a full stop, though.

unable to verify backup file: SHA digest missing from response

is how I think we'd generally word it.