Change CrudRepository to return Optional<T> as return values instead of null reference or provide alternative base interface

As described in the StackOverflow question, I think it would be a big improvement if the repositories returned an Optional<T> instead of possible null references when querying single objects.

Maybe you can consider this when you drop the support for JDK 6 and 7, and plan your next major release. Or you could introduce your own implementation for the Optional<T>, like the guava team did, when not dropping the JDK support for 6 and 7.

This change would only effect some of the CrudRepository interface methods. And the implementation is almost trivial.