necessary adaptions to publish to MavenCentral

[stempler]

Changes needed to deploy the artifact to Maven Central (plus Sonatype Nexus account and registered GPG key being available), see #6.

I used these guides to do the changes:

• http://central.sonatype.org/pages/requirements.html#sufficient-metadata
• http://central.sonatype.org/pages/apache-maven.html#deploying-to-ossrh-with-apache-maven-introduction

I tested it by deploying to Maven Central (with a different group name), as I need it available in JCenter to be able to release a Gradle plugin that depends on it. But it would surely better to have an official version out.