Make the @FAS.postlogin decorator working

This commit allows to have a method in the application with the decorator
@FAS.postlogin. If such decorator exists, flask-fas-openid will call it
after having successfully logged-in, otherwise it will proceed as before.

Signed-off-by: Pierre-Yves Chibon <pingou@pingoured.fr>
Adjust docstring in the login() method

Signed-off-by: Pierre-Yves Chibon <pingou@pingoured.fr>
Merge pull request #85 from fedora-infra/feature/return_callback

Feature/return callback