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
my purpose was it to serve a configuration solution to create own  `AlternateLocaleProvider` and pass its service id through that parameter. As it is done for the route providers/filters in routing bundle. 
But do i understand it right: i should just use the class of`cmf_seo.alternate_locale.provider` means `cmf_seo.alternate_locale.provider.class`. Either somebody overwrites it or not?
So i would just have to look if the service is defined and add it.
But got one problem: That service definition lives in the phpcr namespace/world cause i inject `doctrine_phpcr` - thats the special the CMF own provider. "When you use phpcr-odm and activate `alternate_loacle` you will get an `AlternateLocaleProvider` for free, one that is able t work with the phpcr-odm to get the locales"