Blocks everywhere

By using blocks individual admin classes can add additional functionality to the pages.
Currently there is only an area to the left defined in the standard template but there could be others as well.

The admin classes simply need to implemente the getBlocks method, e.g.
```
public function getBlocks($position)
{
    if ('left' == $position) {
        return array(array('type' => 'sonata_admin_doctrine_phpcr.tree_block'));
    }
}
```