provide route collection hierarchy

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | no
| Fixed tickets | 
| License       | MIT
| Doc PR        | 

I know this repo is obsolete but we still use Symfony 2.1 for our [cms sandbox](https://github.com/prestaconcept/prestacms-sandbox). I've submit the same PR in [RoutingBundle](https://github.com/symfony-cmf/RoutingBundle/pull/96)