Story 6: Develop scaffolding code for managing `User` Entity

[ebouchut]

Story 6: Develop scaffolding code for managing User Entity:

Implement the RESTFUL API for:

1. Creating a User Entity to do the Spring Data Repository
2. Reading a User Entity from the existing Spring Data Repository
3. Updating a User Entity from the existing Spring Data Repository
4. Deleting a User Entity from the existing Spring Data Repository

[ebouchut]

Renamed User entity attributes (see ab5232e) to follow naming conventions (camelCase):

• User.firstname to User.lastName
• User.lastname to User.lastName