Discussion tracking

We have now added tracking on when comments have appeared in view.
There are three instances in which this could be the case:

* You have `#comments` in the URL
* You use a comment permalink (`#comment-ID`)
* You scroll down to the comments and have the container within the top half of your view port

We will be looking at enhancing this stat to include dwell time soon.

![Track win](http://gifling.com/galleries/gifs2/player-dance-on-running-track.gif)