added pdo class feature

This is a port of Yii1 feature https://github.com/yiisoft/yii/blob/master/framework/db/CDbConnection.php#L251. This will also enable such features like custom ```pdoClass``` with nested transaction (it is commonly used in Yii1) https://github.com/yiisoft/yii2/issues/1645.  
I also removed check for those three drivers and added a comment to ```pdoClass``` property about it, because of if this check will be for those db there will be no custom pdo class support as you can see.