Allow easy usage of custom Role class 

To use a custom class inherit from  from AdjacencyList/Role/DoctrineDbal.php and overwrite createRole()  according to your needs. 
```php
class MyProvider extends DoctrineDbal {

  protected function createRole(array $row) {
     $role = new My\Custom\Role($row['name']);
     // Optional: $role->setId($row['id']);
    return $role;
  }

}
```