New: Add statistics report for supplier invoices

htdocs/compta/facture/stats/facturestats.class.php

htdocs/compta/facture/stats/index.php

htdocs/compta/facture/stats/month.php

htdocs/includes/menus/barre_left/eldy_backoffice.php

htdocs/includes/menus/barre_left/eldy_frontoffice.php

htdocs/langs/en_US/bills.lang

htdocs/langs/fr_FR/bills.lang

htdocs/stats.class.php

htdocs/viewimage.php