[WIP] Add block options

This PR adds the feature to overide options like the template. See https://groups.google.com/forum/?fromgroups=#!topic/symfony-cmf-devs/_t0jQCsdEcU I think this could be a solution, it works without BC breaks on the BlockInterface. It is probably far from good, but this will give an idea and could start discussion.

It allows to do things like this in the template:

```
    {{ sonata_block_render({
        'name': 'additionalInfoBlock',
        'template': 'SandboxMainBundle:Block:overloaded_container_block.html.twig',
        'attr': {
            'class': 'additional-info-block'
        },
        'settings': {
            'divisibleBy': 3,
            'divisibleClass': 'row-fluid',
            'childClass': 'span4'
        },
        'children': {
            'symfony_cmf.block.simple': {
                'template': 'SandboxMainBundle:Block:overloaded_block_simple.html.twig',
                'attr': {
                    'class': 'simple-block'
                }
            }
        }
    }) }}
```

Furthermore configuration for a block is split in:
* settings: fe. a url for a feed, this configuration could be changed through a UI
* options: configuration to be used in templates etc. but not changed through a UI