[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
yeah i see.

Would it be better to return an object rather than an array? I think that would define a clearer API

````
$contentQuery = $contentQueryBuilder->build(...); // returns ContentQuery 
$contentQuery->getFields();
$contentQuery->getSql2();
````

or

````
$contentQuery->getQuery(); // PHPCR\QueryInterface / QueryBuilder ?
````

?