retrieve querybuilder with createQueryBuilder from Entity repository

Problem : each method who need a query builder retrieve it by the following `$this->_em->createQueryBuilder()`. 
Whith this way we can't inject our own QueryBuilder (e.g: new PageQueryBuilder($options) to retrieve
a preconfigured QueryBuilder according to entity business (left join, or all what we want)

Use createQueryBuilder method from EntityRepository provided by doctrine make this possible, we just
need to overload her in this case.

In my review in found this weird : ` $qb = $this->_em->createQueryBuilder($meta->name)` on ` lib/Gedmo/Tree/Entity/Repository/MaterializedPathRepository.php` L:87. The QueryBuilder provided by the entityManager does not accept any argument (According to : ` Doctrine\ORM\EntityManagerInterface.php`)