Update RP API: add onready, remove returnTo

There's still some discussion of onready being important, so we'll hang onto it.

As for returnTo, it feels awkward and confusing when considering a world with
native client support for BrowserID:

- Users served by a native IdP will fully authenticate within native dialogs,
  making it easy for the browser to preserve the original page context.

- Users served by a fallback IdP... there doesn't seem to be an obvious way to
  handle returnTo without leaking the RP to the fallback, undermining the
  privacy guarantees of the BrowserID protocol.

(To avoid leaking information to fallbacks, the client could, perhaps, wait
until it saw an Identity Certificate get registered for the given address and
then log the user in... but if you're going to do that, why not just use the
original page context, again obviating the need for returnTo...)