Show relation in list view even if no admin association is set (fixes #31)

When a many to one relation is defined on a Doctrine (MongoDB) Document, the related document is never shown in the list view.

In the following example, category is a property of a Post that contains a Category object. 

```php
    protected function configureListFields(ListMapper $listMapper)
    {
        $listMapper->add('category');
    }
```

I expect the string representation of the category to be shown in the list view. Instead nothing is shown.