Added article sharing functionality.

Allows for sharing of articles to Google+, Twitter, Facebook, and email using share URLs located at the bottom of an article stub/article, just above the Discuss link.
![screen shot 2014-03-09 at 5 11 52 pm](https://f.cloud.github.com/assets/2200898/2369130/92060c2c-a7cf-11e3-82f0-0939ab6a7fa4.png)