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:
0. Add a feature and specs
1. Implement single sign-on, so that users can comment by signing into WSO.
2. Add comments counters to links for docs and articles
3. Think of unique identifiers for docs and resources (currently uses their url, which can change, when doc/article is renamed)
rather than having it as a global constant initialized on startup, could we move this into a module of its own? the module can contain the shortcode and logic to render the partials if necessary. I would feel better accessing the shortcode with `Disqus::SHORTCODE` rather than the global `DISQUS_SHORTCODE`