Proper release procedure for Digger Java client

Why?

We need proper release and publish procedure. Current manual way is a no-go for an open source project.

What?

• Introduce Maven release plugin and do the releases using that.
• Set up Maven to publish to Maven central with a Maven task.
• Check if there is some automated testing in Travis
• Document how release is done, how publishing is done, who to contact to get some access rights, etc.