[WIP] 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
@dbu that's the way i try to create the service depending on the existence of the `doctrine_phpcr` service, but this does not really work, never got into that clause - think it is a time problem in the order of setting up the configs. Tried the same in a `CompilerPass` and with class_exists(\Doctrine\Bundle\PHPCRBundle\DoctrinePHPCRBundle)` both does not work.