Lightbox for article images

Article images now have an icon to launch all of the article's images in a lightbox view:

![screen shot 2014-11-18 at 11 22 30](https://cloud.githubusercontent.com/assets/2236852/5104784/5a49b8f2-6fdd-11e4-845d-da916f88f997.png)
![screen shot 2014-11-18 at 11 22 39](https://cloud.githubusercontent.com/assets/2236852/5104785/5b4ac7a0-6fdd-11e4-8780-be4087c71a10.png)

cc @mattosborn 
use `$gs-baseline` for vertical spacing