Switch from Maven to Gradle

[svetob]

• Build with gradle
• Convert all files to Kotlin including main class
• Make sure .jar and docker-image works

[bergetp]

Isn't converting all files to Kotlin another task? :) Would be sweet to have rabbit-puppy in bintray, which I guess is the main reason you're switching to gradle?

[svetob]

The main class is currently in Java because of some odd maven integration issue that I couldn't figure out... but it works well in gradle :)