Declaring namespaces in the Symfony >=2.1 way

I think the installation documentation should be updated.

Update the autoload.php to add new namespaces:
```php
<?php
$loader->registerNamespaces(array(
    'Sonata'                             => __DIR__,

    // ... other declarations
));
```

This seems to be the Symfony 2.0 way of declaring namespaces. Now we should have
```php
<?php
// intl
if (!function_exists('intl_get_error_code')) {
	$loader->add('Sonata', __DIR__);
	// … other declarations
}
```

My 2 cents

Guillaume