add event-based RP API

[benadida]

I propose moving to an event-based API for web sites.

// this event is guaranteed to fire after window.onLoad
// so it's best if this event listener should be registered before window.onLoad
navigator.id.addEventListener('login', function(event) {
  // do things with event.assertion
});

navigator.id.get({options});

Advantages:

• we do away with allowPersistent and silent, as those become entirely user-controlled now. The only option for that remains for now is requiredEmail.
• firing the login event can be done in a number of cases where the reference to the callback has otherwise been lost, e.g. when returning from having verified an email if the RP page was unloaded.

[lloyd]

suggesting 3 stars given other open issues.

[jbonacci]

Just noting here there is a pretty good discussion with use cases in dev.identity:
http://groups.google.com/group/mozilla.dev.identity/browse_thread/thread/4e19416ef49487b9#

[lloyd]

A proposal which fleshes out ben's original ideas is here: http://lloyd.io/a-new-api-for-browserid

[lloyd]

'Observer' API introduced experimentally train-2012.04.11: https://gist.github.com/2287552