Add a login_required decorator

Added a AnonymousUser class and a is_authenticated method to this and to
be User class to be able to tell them apart.
Add authentication helpers for testing

This means a assert401 method and a AuthenticatedUserMixin class, that
provides a 'authenticate' method that will create a user and assign a
auth_token to self.auth_token.
Restrict access to creating streams to authenticated users

Note that editing and deleting streams is still open to everyone...