JDK 21 Release

Updates to run on JDK 21 LTS.

Also bumped plugin dependencies.

JUnit 5 can wait for another day... ;-)

Full Changelog: 1.3.0...1.5.0

Maintenance Release

1. Updated dependencies.
2. Fix shoddy Javadocs.
3. Added Gradle build.
4. Moved to GitHub Actions for CI.

Bumped JUnit version.

Added Visitor pattern. Thorough cleanup of code and formatting.

Added travis-ci automated build capability to project. See https://travis-ci.org

The first release after mavenising the project.