Improvements to Social Media features

* Closes #225.

This commit includes the following:

* OpenGraph markup for reliable behaviour during Facebook sharing.
* Facebook App markup to provide *some* demographic insight.
* Facebook Like button in footer (likes the [Facebook Page](http://www.facebook.com/theyworkforyou)).
* Twitter Follow button in footer (follows [@theyworkforyou](http://twitter.com/theyworkforyou)).
* Google Analytics tracking of like, unlike and follow events.