A chapter on authentication and authorization

This is a first rough version. There is room for improvement and proof-reading.
Add some stuff to make it nice

 - Show the username when logged in
 - Show a "log out" link when logged in
 - Redirect to the index when logging out

and fix some typo on the way.
Merge pull request #53 from kampfschlaefer/chapter_security_authentication_and_authorization

A chapter on authentication and authorization