Change invoice_number in Receiving to be Reference, tidy up code (#635)

database/database.sql

@@ -441,11 +441,11 @@ CREATE TABLE `ospos_receivings` (
   `comment` text NOT NULL,
   `receiving_id` int(10) NOT NULL AUTO_INCREMENT,
   `payment_type` varchar(20) DEFAULT NULL,
-  `invoice_number` varchar(32) DEFAULT NULL,
+  `reference` varchar(32) DEFAULT NULL,
   PRIMARY KEY (`receiving_id`),
   KEY `supplier_id` (`supplier_id`),
   KEY `employee_id` (`employee_id`),
-  UNIQUE KEY `invoice_number` (`invoice_number`)
+  KEY `reference` (`reference`)
 ) ENGINE=InnoDB DEFAULT CHARSET=utf8  ;
 
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