Add icon for external links

potential fix for #19 

i can twiddle with the icon; it's just wikipedia's external link icon with the colors changed a bit

![2013-03-20-230614_75x33_scrot](https://f.cloud.github.com/assets/873857/284191/6fdc77ec-91d4-11e2-973d-a697fb417f46.png)