Callback subscriber

Added ability to specify callback for pagination.

```php
$pagination = $paginator->paginate(
    new PaginationCallback(function() use ($service) {
        return $service->total();
    }, function($limit, $offset) use ($service) {
        return $service->getAll($limit, $offset);
    }),
    $this->get('request')->query->get('page', 1), 1
);
```