Add decorator to hide non-auth only views.

https://github.com/madewithbytes/us_ignite/issues/57
https://github.com/madewithbytes/us_ignite/issues/56

Some views such as login and registration should only
be available to non-logged in users.

The ``common.decorators.not_auth_required`` decorator
redirects the authenticated user to the homepage.

Closes #57 and #56.