Usagi v0.1

Add Gradle build with shadowJar for a fat JAR

The project builds only through the IDE, so there is no repeatable
way to produce a runnable artefact for someone who has Java but not
this project open in IntelliJ.

Let's add a Gradle build using the shadow plugin, which packages the
classes and any future dependencies into a single runnable JAR at
build/libs/usagi.jar.

The Gradle wrapper is committed alongside it so the build uses a
fixed Gradle version without anyone needing Gradle installed.