Updated trans_start() and trans_complete() so that _trans_depth increments correctly

Fix for issue #159 and #163.
diff --git a/system/database/DB_driver.php b/system/database/DB_driver.php
index dd1b567..cc40ba4 100644
--- a/system/database/DB_driver.php
+++ b/system/database/DB_driver.php
@@ -522,6 +522,7 @@
 		}
 
 		$this->trans_begin($test_mode);
+		$this->_trans_depth += 1;
 	}
 
 	// --------------------------------------------------------------------
@@ -545,6 +546,10 @@
 			$this->_trans_depth -= 1;
 			return TRUE;
 		}
+		else
+		{
+			$this->_trans_depth = 0;
+		}
 
 		// The query() function will set this flag to FALSE in the event that a query failed
 		if ($this->_trans_status === FALSE)