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.)
I've called it RPC since we're calling a client API method remotely (via HTTP).  I considered putting HTTP in the name too, but honestly it's practically implied.