Added a Java8/CompletableFuture example.

Works at least with the following java version:
$ java -version
java version "1.8.0-ea"
Java(TM) SE Runtime Environment (build 1.8.0-ea-lambda-nightly-h4646-20130603-b93-b00)
Java HotSpot(TM) 64-Bit Server VM (build 25.0-b33, mixed mode)