Unobtrusive Registration Support for Devise::Confirmable

If a user wants to enable Devise::Confirmable they have to completely override `Spree::UserRegistrationsController#create`.

The existing `Spree::UserRegistrationsController#create` method:

1. does not call `sign_up` which means there is no easy to define hook to add custom code on signup
2. does not respond to `active_for_authentication?` which is necessary to support :confirmable settings

This branch addresses both. Existing tests pass.