[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
As mentioned previously I think using the query builder would be more reliable than constructing SQL2 queries, both because queries are more likely to contain errors and the JCR SQL2 query parser used by Jackalope is buggy: http://phpcr.readthedocs.org/en/latest/book/query.html#the-query-builder-a-fluent-interface-for-qom