refactoring one ListenableFuture instance to Java 8 CompletableFuture

Hi, I'm doing research on new concurrent constructs in Java 8. I found CompletableFuture in Java 8 has the same functionality as Guava ListenableFuture. But CompletableFuture is much nicer because it comes together with Lambda expression in Java 8, and it is monadic, which makes the code more readable and cleaner. Also, it provides more ways to compose different tasks. Therefore, using CompletableFuture instead of ListenableFuture is better for future extension and maintenance of the code.

I tried to refactoring one ListenableFuture instance to Java 8 CompletableFuture in `atlas`, so you can see the difference. You don't have to merge this pull request. I'm just wondering your opinion on this kind of refactoring (or migrating the code from Java 6, 7 to Java 8). Do you think the refactoring is useful? Do you have any plan to use Java 8? Thanks.