DATACMNS-489 - Support for Future<T> as return value wrapper for repository methods.

Added a converter to process a NullableWrapper into a Future by producing an AsyncResult. This is to support repository methods annotated with @Async and returning a Future<T>. With Spring's asynchronous method invocation support activated this will cause the repository method being executed asynchronously.

Note, that this currently only works on Spring 4 as Spring 3.2.8 async support is not discovering the introduction on the repository proxy and thus fails to discover the @Async annotation on the repository method. This will be resolved in Spring 3.2.9, see the related tickets.

Related tickets: DATACMNS-483, SPR-11725, DATACMNS-499.