Create User Command

Create User Command:

- [x] test coverage (no tests)
- [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?   | none
| Fixed tickets | none
| Doc        | https://github.com/massiveart/sulu-docs/blob/master/developer-documentation/400-contacts/create-user.md