[FEATURE] implemented method to get all users in system

implemented method to get all users in sulu system and tests

__Tasks:__

- [x] implement repository method
- [x] test coverage
- [x] gather feedback for my changes


__Informations:__

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| Tests pass?   | no --> needs to be fixed https://github.com/sulu-cmf/SuluSecurityBundle/pull/67/files#diff-9f1a6390c5918f23b45fadf5ffcdf3ebR210
| Fixed tickets | none
| Doc           | none