Display when User is logged in
Enable User to log out
Log new users in after the registered
Link to Log In form
Forbid access to non-authenticated users
Hints
Only validate user password on create

The password should be validated again, when it is changed!
Fix Link text
Prepare App to send mails and to display them...

... in the browser in development, through the letter_opener gem
Merge commit '99b701a211' into day2
Merge commit 'ff5aba0d86' into day2