DATAREST-421 - Extracted @ControllerAdvice class in order to allow easy overriding.

Original pull request: #155.
DATAREST-421 - Polishing of new exception handling infrastructure.

Removed leftovers from AbstractRepositoryRestController. Renamed GlobalExceptionHandler to RepositoryRestExceptionHandler and minimized visibility of exception handler methods. Restricted application of the exception handler to controllers in the Spring Data REST base package.

Relatted pull request: #155.