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.)
Actually, having another look it made sense to pull this out.  That's particularly the case since time.Time.Format() can't handle the format we want.