* i18n
* Vérification des variables dans les classes + throw exception
* tokens + ban system
* remember me
* htmlspecialchars => on users objects
* Associate a guest with someone

install.php :
=============
* Link beside password field to toggle visible / not visible
* 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)

index.php?do=new_invoice :
==========================
* Improve date handling for form display

index.php?do=settings :
=======================
* Prefill the timezone field
* Fill the fields with POST
* Handle checkboxes in PHP ?
* JavaScript to handle singular / plural + months