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)

If you're only transitioning 1 property you probably don't need to use `all` here, but bourbon doesn't handle it very nicely. Up to you, really

```
@include transition-property(transform);
@include transition-duration(0.15s);
@include transition-timing-function(linear);
```