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.)
So it should look like this (assuming you meant "environment" rather than "environ")?

url := fmt.Sprintf("%s/environment/%s/%s", c.st.serverRoot, uuid, method)

From where do I get uuid?