The REST API methods for entity creation and modification have been
simplified with more basic entities.

In addition, the UserResource class has been added so that it is
possible now for clients to use it for user authentication.

The REST layer has been added but with incomplete tests. Students will
have to complete the tests.

This commit includes an important refactorization regarding tests. Test
classes and datasets have been moved from the "service" project to a new
"tests" project. This way, any project can include this "tests" project
as a dependency in order to have access to these common testing
resources.