add support for Query Hints

Eg with a NULLS LAST Walker:

    $builder = new TableBuilder($entityManager, $_GET);

    $builder
        ->from('Foo\Bar\Entity\Sample', 's')
        ->add('text', 's.name')
        -> ... more stuff ...
        ->setHint(Query::HINT_CUSTOM_OUTPUT_WALKER, 'FooBundle\Doctrine\Extensions\Query\SortableNullsWalker')
        ->setHint('SortableNullsWalker.fields', array('created_at' => SortableNullsWalker::NULLS_LAST))
        ;