Change sessions to be stored in database and add SQL upgrade script 2.3.4 to 2.4 (#120)

database/2.3.4_to_2.4.sql

@@ -0,0 +1,10 @@
+-- alter sessions table
+DROP TABLE `ospos_sessions`;
+
+CREATE TABLE `ospos_sessions` (
+  `id` varchar(40) NOT NULL DEFAULT '0',
+  `ip_address` varchar(45) NOT NULL DEFAULT '0',
+  `data` blob,
+  `timestamp` int(10) unsigned NOT NULL DEFAULT '0',
+  PRIMARY KEY (`id`)
+) ENGINE=InnoDB DEFAULT CHARSET=utf8;