sinon.spyMultiple
make default: clear first
Refactor registration and authentication form eventification

Mixing UI-related tihngs with event emitting didn’t feel right, so I:
- extracted RegistrationHandler out of the authenticate controller method in
  registration form directive
- deifined XO.PromiseHanlderInterface which AuthenticationHandler and
  RegistrationHandler implement
- extracted XO.EventifiedPromiseHandler -- a decorator to hold eventification
  code for AuthenticationHandler and RegistrationHandler