JS bindings for session registration

[drubery]

One thing that's challenging about the current header-based API is that you can't know when registration has completed. We should design some JS bindings that let sites start a registration and wait for it to complete. This would be an extension of the functionality requested in #117.

It's not clear today how much of the header-based functionality should end up in the JS bindings. There has been some concern about exposing authorization/challenge to JS. The easiest thing might be a wrapper around the header-based API that:

• Takes an endpoint parameter and returns a promise
• Fetches endpoint, expecting to get a Secure-Session-Registration header in the response
• Doesn't resolve the returned promise until registration actually completes