Map settings property + merge with configuration in PHPCRBlockLoader

The result is that settings are cascaded:

* default settings are stored in the block service
* settings can be altered through template helpers:

 ```
{{ sonata_block_render({  
      'type': 'symfony_cmf.block.service.rss',  
      'settings': {  
          'title': 'Symfony2 CMF news',  
          'url': 'http://cmf.symfony.com/news.rss'  
      }  
  }) }}
```

* and settings can also be altered in a block document