Add settings to container block

Together with symfony-cmf/BlockBundle#10 this gives more control on how a container block and its children blocks are rendered.

An example for the Sandbox would be:

```
{# SandboxMainBundle:Homepage:index.html.twig #}
{{ sonata_block_render({
    'name': 'additionalInfoBlock',
    'settings': {
        'divisibleBy': 3,
        'divisibleClass': 'row'
    }
}) }}
```