[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)

__Dependend PRs:__

* https://github.com/sulu-cmf/sulu/pull/229
* https://github.com/sulu-cmf/SuluTestBundle/pull/23

__Informations:__

| Q             | A
| ------------- | ---
| Tests pass?   | no => because https://github.com/sulu-cmf/sulu/pull/229 , https://github.com/sulu-cmf/SuluTestBundle/pull/23
| Fixed tickets | none
| BC Breaks     | none
| Doc           | TODO
Again this would all be unnessecary when we refactor .. e.g. `$workspace->query` could return `Structure` objects from `$row->getNodes`. The `Structure` objects could then be cast to an array if necessary.