Allow relative urls in the IdentityProvider.register() call

[samuelgoto]

I ran into this issue while trying to put a local server up, taking notes so that it doesn't fall through the cracks.

During development, it is useful to use relative URLs (say, "/config.json", rather than "https://idp.example/config.json") in the register() call so that you can have a local server listening to localhost. The current implementation throws an Error: cross-site configURL when a relative URL is passed.