Created class Resources that defines @Produces methods to Inject them in code. For now, there is only one method for injecting slf4j Logger.
Replaced static access to slf4j Logger by @Inject Logger logger.
Created ServiceException to wrap dao and other excetions thrown in service layer.
Created interface CommentService and implemented CommentServiceImpl using hibernate bean validation.
Created tests for CommentServiceImpl using hibernate Validator class.
Deleted constraint on Comment.id to use it correctly with hibernate validator.

Signed-off-by: Radek.Koubsky <radekkoubsky@gmail.com>
Merge branch 'master' of https://github.com/halafi/pv243-music-manager.git