Declaration of <newVersion/> and <oldVersion/> should be optional

[siom79]

The maven plugin should be able to automatically determine and resolve the new and old version.

The new version can be retrieved from MavenProject.getArtifact() while the old version can be computed using a VersionRange query.

This circumvents problems with the ${project.packaging} variable when declaring the maven plugin in the top level pom.xml. For the packaging type maven-plugin the artifact's file type is jar and not maven-plugin as the value of ${project.packaging} would suggest.

[siom79]

Implemented.