* i18n
* handle negative amounts

inc/Invoices.class.php :
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* 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 :
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* TODO : Payback system

TODO :
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* Add / Edit a bill

Tests :
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* Remember me ?

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