Move TouchID signup screen to its own controller.
Refactor how controller is created.
Refactor UI ViewControllers.

Make every VC to have a fixed height so it can be centered in a container.
A0AuthenticationViewController nows only centers vertically the child VC
instead of matching margins.
Add "Already have an account" button.

Pass along authentication parameters.
Make authentication parameters property have `copy` instead of `strong`
modifiers.
Handle login and register of public key.