Add example sites portion of landing page

* Add color variables for example sites section
* Update styles of example sites
* Remove unused SVGs and use PNGs instead

![middleman-example-sites](https://cloud.githubusercontent.com/assets/816402/4911044/e79764d6-6487-11e4-97e6-9b233331428f.png)

A better name for this might be example-sites-header? when I first saw this I assumed it was the title for one of the sites