[FEATURE] REST API Refactoring

__Tasks:__

- [x] test coverage
- [x] gather feedback for my changes
- [x] submit changes to the documentation
- [ ] ... <add your own tasks>

__Informations:__

| Q             | A
| ------------- | ---
| Tests pass?   | yes
| Fixed tickets | #113 
| Doc           | https://github.com/sulu-cmf/docs/commit/0f63e870422987684e2676855931e2ff24a8130f
@chirimoya Would it be ok to outsource this activity to the controller? The implementation here is coupled to `Doctrine\Common\Collections\Collection`, and I can't think of a common solution. If we remove this part the passed entities just have to be iterable.