Add support for more powered-by banners

Hi,

could you add support for more powered-by banners?
You can find an example on the dom4j website ( http://www.dom4j.org ) where you can see some other powered-by banners above the maven logo.

I'll upload a patch that implements this. The only thing you have to do is add a powered-by elements to the navigation.xml file. For example: for the dom4j website, I've added this to navigation.xml:

<powered-by>
<item name="Code Coverage by Clover"
href="http://www.cenqua.com/clover"
img="http://www.cenqua.com/images/clovered1.gif"
/>
<item name="Hosted by SourceForge"
href="http://www.sourceforge.net"
img="http://sourceforge.net/sflogo.php?group_id=16035&type=1"
/>
</powered-by>

thanks,
Maarten