Add sonata caching

This PR applies the block caching implemented in the SonataPageBundle to the CMFBlockBundle.

For testing you could configure caching for the container block on the homepage like this:

* Add sonata cache: ``php composer.phar require sonata-project/cache-bundle``
* Add to AppKernel: ``new Sonata\CacheBundle\SonataCacheBundle(),``
* Add to routing.yml: 
```
    block_cache:
        resource: "@SymfonyCmfBlockBundle/Resources/config/routing/cache.xml"
        prefix: /
```

* Update config.yml: 
```
    sonata_block:
        # ...
        blocks:
    	    # ...
            symfony_cmf.block.container:
                cache: symfony_cmf.block.cache.js_sync 
                #cache: symfony_cmf.block.cache.js_async             
```