* Rename fields in Sondage table
* Add field "editable" which tells when users can edit their votes
* PDO : Configure 2 options (FETCH_OBJ + ERRMODE_EX)
* "formatsondage" does contains "+" (or not) anymore
* Store all informations about forms into an object "Form" stored in $_SESSION['form']
* Replace connection to database by PDO object
* Check if database is ready in bandeaux.php file