Adding Disqus commenting engine

PT story https://www.pivotaltracker.com/story/show/63446652

I only wanted to make a spike, but this turned out to be ridiculously easy - sign up at Disqus.com and insert their javascript into the view that we want to have comments.

The generic javascript resides in a partial, that can be included into any view.

Basic functionality is in place (i.e. commenting, anti-spam)

TODO:
1. Implement single sign-on, so that users can comment by signing into WSO.
2. Add comments counters to links for docs and articles