Automating parts of publication steps

[flanaras]

Calling the Gradle task publishToSonatype creates a new staging repository. This needs to be closed. In the process of closing the repository Sonatype runs a dozen of validations, e.g. that the JAR files are signed and that Javadocs are published. Once this is done, the staging repository needs to be released.

At the moment, it is documented how to do the last two steps manually. They could be automated, if we want to. See publish-plugin/README.md.

Related #74