Implemented UserManager

Discussion: https://github.com/sulu-cmf/SuluContentBundle/pull/17#discussion_r8694209
Resulted in this PR

- [x] finish the code
- [ ] gather feedback for my changes
- [x] submit changes to the documentation


| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| Tests pass?   | yes
| Fixed tickets | none
| Doc           | none