Methods for converting FeedbackEntity added
Added FeedbackManager; updated FeedbackService
Removed unnecessary code, updated conversion code
Processed comments by teungis
Added new custom exception 'NotFoundException'
Removed unused import, fixed error, added includes

Processed the comments of both teugis and daanvc.
Merge pull request #27 from jorisvervuurt/master

Pull request for implementation of FeedbackService.