[FEATURE] Implemented Sitemap

__Tasks:__

- [x] test coverage
- [ ] gather feedback for my changes
- [ ] submit changes to the documentation

__Known Bugs:__

* for multiple Portal urls first will be used
* shadow pages (https://github.com/sulu-cmf/SuluWebsiteBundle/issues/40)

__Informations:__

| Q             | A
| ------------- | ---
| Tests pass?   | yes
| Fixed tickets | none
| BC Breaks     | none
| Doc           | TODO
I think we should be prefixing all sulu twig extensions with `sulu` this will ensure that we avoid conflicts with other packages that users may choose to install. e.g. `sulu_sitemap_url`.