restructuring project for gradle build

Hey oyachai,

Your hearthstone project looks great! I've done some refactoring to make it easier for anyone to get it up and running locally. It introduces Gradle as a build tool, which makes it incredibly simple for someone to build the project, setup their IDE etc.

just run ./gradlew test and watch all the tests run (first run will take a moment as dependencies download).

I also switched JSON to use a library and imported the IdentityLinkList as not everyone has that in their JVM.

I'd love to help contribute; let me know what you think about this change!