* Don't cache the username
* JSON output => do index view
* API
* Reattribute all invoices / paybacks to unknown user when deleting a user

inc/Invoices.class.php :
========================
* Add / Edit a bill

Manage paybacks :
=================
* TODO : Payback system (class should be ok)
* TODO : Global payback

To test :
=========
* Connection form
* Remember me ?
* Edit notice
* Add / Edit user
* Change password
* Change settings