Add information about CompleteableFuture

[CompleteableFuture](http://download.java.net/lambda/b88/docs/api/java/util/concurrent/CompletableFuture.html) is way better than Guava's [ListenableFuture](http://docs.guava-libraries.googlecode.com/git/javadoc/com/google/common/util/concurrent/ListenableFuture.html). Add a section on futures.
