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

install.php :
=============
* TRUNCATE before CREATE TABLE in install.php

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

index.php?do=new_invoice :
==========================
* Improve date handling for form display
* Onchange not working as wanted
* Handle date/months

index.php?do=settings :
=======================
* Prefill the timezone field

Manage paybacks :
=================