Switch to Gradle build and JUnit (or other) tests

Hi Mahmoud - I just got your book on early release from O'Reilly. I like it so far, and was interested in whether I can help you improve on your examples. This merge would add Gradle build system support. This includes dependency management like Maven, which would mean you could drop the jars in your /lib. Take a look at the build.gradle file, and you'll find it's much more compact than anything you can do in Ant, as Gradle and Maven favor convention over configuration.

You can run the chap 5 test now by running (*nix):
$ cd <path>/data-algorithms-book
$ ./gradlew clean test

Windows:
> gradlew.bat clean test

gradlew is a gradle wrapper that will pull down its own dependencies the first time, then run much faster, so that you don't have to manage gradle installation locally, if you don't want to. 

Let me know what you think. 

- Scott