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.)
The question is, if someone passes in a filename, what would they expect to happen?  You're right that the safest bet would be to fail if the file already exists.  I'll check it out.