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)
well this approach is open to debate, should we use the new Rspec syntax? we could just stick with `shared_examples` which I am happy with, any particular reason you chose `Rspec.shared_examples`?