added session sql instructions for App Engine compatibility

db/session.sql

+CREATE TABLE IF NOT EXISTS `ci_sessions` (
+        `id` varchar(128) NOT NULL,
+        `ip_address` varchar(45) NOT NULL,
+        `timestamp` int(10) unsigned DEFAULT 0 NOT NULL,
+        `data` blob NOT NULL,
+        KEY `ci_sessions_timestamp` (`timestamp`)
+);
+
+ALTER TABLE ci_sessions ADD PRIMARY KEY (id);
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