Add a post authenticate hook

Sometimes it is really useful to have a callback when a user is authenticated. It is triggered all times the user is authenticated no matter if he just logged in or is authenicated from the session cookie.

I have to provide a permission service which gets some data from the authenticated user. So it's not enough using the postLogin hook. 

Using the `initializeSystem` hook and call `User::authenticate` by my own is a bad idea, because it would force every script to require a logged in user.