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 don't think any of the rails conventions mention having a `app/lib` directory. The `/lib/` directory, AFAIK, is specifically for any kind of custom code that doesn’t belong under controllers, models, or helpers.

What is the problem against having it as a global constant that is initialized at startup. Constants like the disqus shortname should be in the initializers (according to Rails convenntion).

Again, with the views, since the disqus partial will be shared among multiple views in multiple resources, they should be kept in the `app/views/shared` directory.