Add a Gradle-based build

[pettyalex]

Add a basic Gradle build, automating the build process to create an executable jar.

This should make it easier and more approachable for people to quickly check out and run this project. I can update the documentation as well if you're interested in merging this.

This allows the project to be built and run through ./gradlew run on UNIX-y platforms or ./gradlew.bat run on Windows, or packaged into an executable jar through ./gradlew assemble.

[pettyalex]

Thanks for the interest! I'll shoot you an email. There's no urgency to this PR, I just wanted to contribute it upstream. I ran into a weird edge case running hap-ibd in the All of Us Environment where we wanted to be able to add some additional logging, and setting up Gradle was a quick and easy way to make iterating and tweaking hap-ibd easier.