[MJAVADOC-376] make the javadoc classifier customizable via property maven.javadoc.classifier

This should work just like my other PR #13 for, just there's unrelated test failures (might because it relies on certain settings).

```xml
<properties>
        <maven.javadoc.classifier>javadoc-jdk6</maven.javadoc.classifier>
</properties>

<build>
        <plugins>
                <plugin>
                        <artifactId>maven-javadoc-plugin</artifactId>
                        <version>2.9.2-SNAPSHOT</version> <!-- local patched snapshot version -->
                </plugin>
        </plugins>
</build>
```