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.)
What would be the benefit of pulling that out into a helper function?  FWIW, I'd probably agree if the 2 lines weren't in their their own block.