* i18n
* Vérification des variables dans les classes + throw exception
* htmlspecialchars => on users objects
* handle negative amounts
* Refactor load method to avoir load_* methods !
* Test remember_me

inc/Invoices.class.php :
========================
* Better way to store users in ? => reprendre cette partie
* Modify load() method to handle complex queries (such as WHERE date < DATE_1 AND date > DATE_2)
* Buyer as user object ?

Manage paybacks :
=================
* TODO : Payback system

Tests :
=======
* Remember me ?
* Add a bill
* Edit a bill

Tests passed (quick tests) :
============================
* Connection form
* Edit notice
* Add / Edit user
* Change password
* Change settings