added git.branch.classifier

added "git.branch.classifier" property that defaults to "" (empty string) for "master" but is otherwise equal to git.branch. Therefore, it can easily be used as classifier:

    <plugin>
      <artifactId>maven-jar-plugin</artifactId>
      <version>${maven-jar-plugin.version}</version>
      <configuration>
        <classifier>${git.branch.classifier}</classifier>
      </configuration>
    </plugin>