Refactoring of errai-jpa-demo-basic sources

Refactoring of all sources of errai-jpa-demo-basic. This refactoring should serve as an example how all other demos in Errai will be treated in the very near future.

Since it is only a preview, it is not absolutely necessary to merge this pull request but this code is already more mature from the coding style and formatting policy point of view (followoing JDF guidelines) and it is already production ready-to-go.

Discussion is welcome.