Add Configuration Tree building support.

So this is my version of TreeBuilding implementation. Let's take an exemple :

```php
<?php

namespace App;

use Knp\RadBundle\AppBundle\Bundle;
use Symfony\Component\Config\Definition\Builder\NodeParentInterface;
use Symfony\Component\DependencyInjection\ContainerBuilder;

class App extends Bundle
{
	public function buildConfiguration(NodeParentInterface $rootNode)
	{
		$rootNode
			->children()
				->scalarNode('Foo')
					->defaultValue("bar")
					->info('This is a fuck*** bar !!')
				->end()
			->end()
		;
	}

	public function buildContainer(array $config, ContainerBuilder $container)
	{
		if (isset($config['foo'])) {
			$container->addParameter('some.foo', $config['foo']);
		}
	}
}
```

This is a default Bundle called "App" with an implementation of my version of TreeBuilding simplification. Of course, we can erased this two method from this bundle. This PR don't break the compatibility. Just add the TreeBuilder support.