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
+1 for config tests in
https://github.com/symfony-cmf/SeoBundle/blob/master/Tests/Unit/DependencyInjection/CmfSeoExtensionTest.php
:-)