Add(GsonTransport): skeleton for a Transport implementation.
Add(UserRegistrationServiceTest).

Refactor(UserRegistration): convert to interface and implementation.
Add(GuiceInterceptorsModule).
Modify(GuiceServletModule): move interceptor bindings to GuiceInterceptorsModule.

Add(ValidationErrorInterceptor).
Modify(UserRegistrationService): remove try-catch clauses.

Modify(UserRegistrationServiceTest): implement test cases for a successful registration and for an unsuccessful one with a UserNameExistsException.