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.)
You'll get a panic here if client.Backup errors out . Handle err as soon as you get it: move c.Assert(err, gc.IsNil) to line 93.