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)
I would prefer to use `FactoryGirl.build(:article)` here (I am not a big fan of mocks or doubles). For example, it will be hard to detect regression when `title` becomes an invalid attribute of `Article` because the test `double` simply stubs methods even when they don't exist.