Added showcase for JDK 8's Optional support on repository methods.

Abbreviated JPA sample module folder names. Renamed the Java 8 auditing sample module to Java 8 module as it not only contains samples for auditing anymore.

jpa/java8/README.md

@@ -0,0 +1,23 @@
+# Spring Data - Java 8 examples
+
+This project contains samples of Java 8 specific features of Spring Data (JPA).
+
+## Support for JDK 8's `Optional` for repository methods
+
+```java
+interface CustomerRepository extends Repository<Customer, Long> {
+
+  // CRUD method using Optional
+  Optional<Customer> findOne(Long id);
+
+  // Query method using Optional
+  Optional<Customer> findByLastname(String lastname);
+}
+```
+
+* `CustomerRepository.findOne(Long)` effectively "overrides" the default `CrudRepository.findOne(…)` to return `Optional.empty()` instead of `null` in case no uniqe element satisfying the query can be found.
+* `CustomerRepository.findByLastname(…)` does the same for a normal query method.
+
+## Support for JDK 8's date/time types in the auditing sub-system
+
+* `Customer` extends `AbstractEntity` which contains fields of JDK 8's new date/time API and those can be populated the same way legacy types like `Date` and `Calendar` can.