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.)
While I'm glad you pointed out Check() (I didn't know), isn't Assert() still correct here.  If I got an error then I have no expectations regarding the filename.