implementing alternate languages for content documents

| Q             | A
| ------------- | ---
| Bug fix?      | [no]
| New feature?  | [yes]
| BC breaks?    | [no]
| Deprecations? | [no]
| Tests pass?   | [no]
| Fixed tickets | [#166]
| License       | MIT
| Doc PR        | not yet

I started to implement that feature. Thought we should take care for the existence of the `ManagerRegistry` and when it exists the `SeoPresentation::setAlternateLocales()` method will figure out urls (by `UrlGenerator`) and pass that information to sonatas `PageService`. By doing it that way the user just needs to add the right TwigHelper `{{ sonata_seo_lang_alternates() }}` to see the result. Same procedure as all other metadata comes into the template.

Todo:
* [x] implement logic in `ContentListener`
* [x] add registry by compiler pass
* [x] add method in `SeoPresentation` to compute and set the alternate lang values to `PageService`
* [ ] tests for unit and frontend
* [ ] take care on current route
* [ ] fix strange url problem
i think you need to check here if the CmfRoutingBundle is available (as we do not require it in composer.json, but the PhpcrAlternateLocaleProvider is needing it).

composer.json should start to suggest routing-bundle as well.