Added support for Java System Tray - needs Java 6

I've also added support for Java System Tray notifications, provided the extension is run on Java 6.

![maven-desktop-notifier-java](https://f.cloud.github.com/assets/170911/1368998/05dbf8b0-39cf-11e3-8680-c0e43a41dc83.png)
![maven-desktop-notifier-java-failure](https://f.cloud.github.com/assets/170911/1368999/05dcd4f6-39cf-11e3-9415-9535905726ec.png)