Add navigation icon support for www.xing.com

Hey Alex, 

thank you for making such an awesome theme. This PR adds support for the German social network XING (comparable to Linkedin) to the navigation bar. Fontawesome already supports the icons for XING. 

If that's too country specific for you, that's also ok for me. I would than make the modification only locally in my octopress installation, but I thought I might as well share it :-)

Cheerio
Björn