[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
Should we start calling a spade a spade? i.e. `$content` => `$structure` ?