[proposal/idea] Fixes #2517: ability to reverse DB criteria order merge.

Fixes #2517. How to use it:

```php
class ActiveRecord extends CActiveRecord
{
	public function getDbCriteria($createIfNull=true)
	{
		if(($criteria=parent::getDbCriteria($createIfNull))!==null)
			$criteria->reverseOrderMerge=true;
		return $criteria;
	}
}

class Test extends ActiveRecord
{
	public static function model($className=__CLASS__)
	{
		return parent::model($className);
	}

	public function scopes()
	{
		$ta=$this->tableAlias;
		return array(
			'order1'=>array(
				'order'=>$ta.'.order1 DESC',
			),
			'order2'=>array(
				'order'=>$ta.'.order2 DESC',
			),
		);
	}
}

$criteria=new CDbCriteria(array(
	'scopes'=>array('order1','order2'),
));
$models=Test::model()->findAll($criteria);
```

@creocoder, your opinion on this?