#6 There are some changes here :
 - I rethought the way I would manage the authentication, adding the
   Control class in the Controllers. I will use the same class to define
   right access for different roles.
 - From now on, a User can login or logout at will, regarding the fact
   that he has the right login & password
 - I added some Javascript code to update the loin/logout link according
   to whether there is currently a User session or not

   P.S. I should have split this commit into different ones. I realize
   that now, it won't happen again.