UserRegistrationUseCase#register: initializes the account data structures
FirebaseDataService: don’t do the 3-way data-binding

Having FirebaseDataService doing the 3-way data binding was coupling it to
Angular and AngularFire. We’ll move this work to the Web UI, where Angular and
friends live.
Authentication form: auto-focus the email field
Chai plugins: housekeeping

- introduced expect(thing).to.be.domElement()
- switched from “new chai.Assertion” to “expect”
- fixed “focused” method to be cross-frame
Authentication and registration forms: focus email field automatically
Logout button: housekeeping
Acceptance tests: authentication: log out