Add a build.gradle file

[rahulmutt]

Eta now has a fully functional Gradle plugin that works well with cross jvm language projects like this one that appears to use Java, Clojure, and Eta.

Given that the current build is not portable since it relies on a local maven repository, it seems like transitioning to a Gradle setup can make things easier.

I'd be happy to provide any assistance if you face any issues in the migration.