Fixed render problem.

Hi, Benjamin.
It is a awesome library.

There is a render problem. The ```porpaginas_render``` displays plain html code.

![render_problem](https://f.cloud.github.com/assets/648770/2030891/275ce9a0-88fc-11e3-84e5-39acc8b39872.png)

https://github.com/whiteoctober/WhiteOctoberPagerfantaBundle/blob/master/Twig/PagerfantaExtension.php#L44
https://github.com/KnpLabs/KnpPaginatorBundle/blob/master/Twig/Extension/PaginationExtension.php#L38