Added possibility to pass custom database objects to DB Forge and DB Utilities

Also, their  property is no longer public and the utility class no longer extends CI_DB_forge.
diff --git a/system/database/drivers/mysql/mysql_forge.php b/system/database/drivers/mysql/mysql_forge.php
index ea32077..019f6d3 100644
--- a/system/database/drivers/mysql/mysql_forge.php
+++ b/system/database/drivers/mysql/mysql_forge.php
@@ -74,11 +74,12 @@
 	/**
 	 * Class constructor
 	 *
+	 * @param	object	&$db	Database object
 	 * @return	void
 	 */
-	public function __construct()
+	public function __construct(&$db)
 	{
-		parent::__construct();
+		parent::__construct($db);
 
 		$this->_create_table .= ' DEFAULT CHARSET '.$this->db->char_set.' COLLATE '.$this->db->dbcollat;
 	}