[FEATURE] Implemented content query

Content Query used to build queries for loading small content pieces.

__Tasks:__

- [x] test coverage (Issue: https://github.com/sulu-cmf/sulu/issues/264)
- [ ] gather feedback for my changes
- [x] submit changes to the documentation (Issue: https://github.com/sulu-cmf/sulu/issues/264)


__Informations:__

| Q             | A
| ------------- | ---
| Tests pass?   | yes
| Fixed tickets | none
| BC Breaks     | none
| Doc           | TODO
Can we not retrieve an extension by key instead of iterating?