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)
The response is not mocked up.  
The phantomJS queries Disqus.  The comments are just warning that Disqus account has to be set-up and configured for this.
Stubbing a response is a possibility for the future, if we use the PuffingBilly gem.  But we need to test how it behaves on travis.