implement registration and activation email

- implement (actually copy) methods clean_username and clean_email in
  caffeine.forms.CoffeestatsRegistrationForm
- implement caffeine.models.CaffeineUserManager with methods create_user and
  create_superuser
- use CaffeineUserManager as manager for caffeine.models.User
- drop registration_complete from caffeine.urls and use
  CaffeineRegistrationView as registration_register
- remove registration_register from coffeestats.urls
- implement caffeine.views.CaffeineRegistrationView and redirect to home page
  in get_success_url
- remove unused caffeine.views.RegistrationCompleteView
- coffeestats.settings.base: get SECRET_KEY from environment variable
- add styling for ul.errorlist in static/css/caffeine.(s|)css
- add form error messages in templates/registration/registration_form.html