Supported for using referrer fields in queries

It turns out that it was not possible to use referrer fields in the query builder, this PR fixes that, and makes joining on referrers possible.

Closes: https://github.com/doctrine/phpcr-odm/issues/467


````php
        $qb = $this->dm->createQueryBuilder();
        $qb->fromDocument('Doctrine\Tests\Models\CMS\CmsUser', 'u');
        $qb->addJoinInner()
            ->right()->document('Doctrine\Tests\Models\CMS\CmsAddress', 'a')->end()
            ->condition()->equi('u.address', 'a.uuid');
        $qb->where()->eq()->field('a.city')->literal('Lyon');
        $q = $qb->getQuery();
        $res = $q->execute();
````