Social most popular

Tests displaying a most popular to infrequent users containing content which has been most highly referred from social media. (See shot below) 

Server-side: 
  *Adds a social-referrals call to ophan
  *Adds an agent to retrieve content from ophan hourly 
  *Adds a socially referred content endpoint

JS: 
  * Extends history API to provide ( crude ) session count over time
  * For the test variant, displays the socially referred content to the user and hides the usual most-popular component

![social-referrals](https://cloud.githubusercontent.com/assets/986155/3233747/8ace6644-f0c1-11e3-9af4-85fc468b3b71.png)


`Nil` is a nice replacement for `Seq.empy` (`Nil` is an empty list)