[MJAR-170] make the jar classifier customizable via property maven.jar.classifier

tested with the following snippet in my pom.xml and it works:

```xml
<properties>
        <maven.jar.classifier>jdk6</maven.jar.classifier>
</properties>

<build>
        <plugins>
                <plugin>
                        <artifactId>maven-jar-plugin</artifactId>
                        <version>2.5-SNAPSHOT</version> <!-- local patched snapshot version -->
                </plugin>
        </plugins>
</build>
```