#1515  Use index support in relations

Use index support in relational queries. 
Usage :

```php
  $criteria = new CDbCriteria();
  $criteria->with =array('products' => array('joinOptions' => 'USE INDEX(someindex)'));
  Category::model()->findAll($criteria);
```
in ActiveRecord ->relations() :
```php
public function relations()
	{
		return array(
			'parentCategory' => array(self::BELONGS_TO, 'Category', 'parent_category_id'),
			'products' => array(self::HAS_MANY, 'Product', 'category_id','joinOptions'=> 'USE INDEX(someindex)')
         );
    }

}
```