[FEATURE] Reorder node manually

Implemented orderBefore method in content mapper:

__Tasks:__

- [x] test coverage
- [ ] gather feedback for my changes
- [x] submit changes to the documentation
- [x] merge depending PR (https://github.com/sulu-cmf/sulu/pull/169)
- [x] order-beofre in content-mapper
- [x] UI (https://github.com/sulu-cmf/SuluContentBundle/pull/174)

__Informations:__

| Q             | A
| ------------- | ---
| Tests pass?   | yes
| Fixed tickets | https://github.com/sulu-cmf/sulu-standard/issues/124
| BC Breaks     | no
| Doc           | https://github.com/sulu-cmf/docs/commit/f8202a35449045a6c36516e31a2af30f30de0ecb
@dantleech :+1: