Add Disqus support (opt-in)

I thought this might be useful for the one or two. 

If you want to enable Disqus comments, simply remove the `-->` in line 73 and append it on line 56. Last but not least, adjust `%YOURDISQUSNAME%` to your Disqus shortname.

You can find a demo of this [here](http://blog.frd.mn/the-centos-disc-was-not-found-in-any-of-your-drives/). 

PS: Make sure you've set the "Color scheme" of Disqus "for light backgrounds" in Settings -> General -> Appearance:
![bildschirmfoto 2013-10-28 um 11 35 22](https://f.cloud.github.com/assets/944459/1418749/fd19d276-3fbc-11e3-9f0e-29c780f6c63b.png)