feat(client): Seemless verification in a second browser.

* Create a `resume` token from OAuth parameters. Pass it along to the fxa-js-client.
* Re-populate OAuth parameters from resume token on email verification.
* Bump fxa-js-client to 0.1.25 with the new `resume` hotness.
* Add functional tests for verifying in a second browser.
* Only show the serviceName in the ready screen if the user can continue with the OAuth flow.

Introduce the User model.
* Create the User instance in app-start.js. Pass it to the router which passes it to the Views.
* Update the complete_sign_up View to make use of the User model.
* Update the OAuth Relier to set the email on the User model instead of itself.

issue #1623