* i18n
* handle negative amounts

inc/Invoices.class.php :
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* Modify store() method to handle storage
* Modify load() method to handle JOIN
* Modify load() method to handle complex queries (such as WHERE date < DATE_1 AND date > DATE_2)
* Cache

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

TODO :
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* Add / Edit a bill
* JSON output
* Arrow to toggle visible password

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