Publish Incrementals (JEP-305)

[sghill]

The gradle-jpi-plugin does not automatically configure the Jenkins Incrementals repository as specified in JEP-305.

The simplest way to support this feature would be defining the repository according to Gradle's built-in maven-publish plugin. This will create tasks for publishing to a specific repository that can be invoked when appropriate.

#214 is a prerequisite.

This is an action item from #213.