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.)
Good point.  Would it hurt to stick somewhere common, like testing/helpers.go.  That's where I'll move it for now, but if you have a better suggestion I'm all ears. :)