[FEATURE] integrated categories for accounts

- added account category entity and API
- added tests and adjusted test of accounts
- integrated category selection / overlay to choose / add and edit categories
- integrated data mapper

__Tasks:__

- [x] create orm xml and entity
- [x] create rest controller
- [x] implement tests
- [x] integrate select and overlay
- [x] test coverage
- [ ] gather feedback for my changes

__Informations:__

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| Tests pass?   | yes
| Fixed tickets | none
| Doc           | none

merge https://github.com/sulu-cmf/SuluAdminBundle/pull/109 before this PR