Add wireframe design and base HTML structure for exercise

* Adjust base grid settings to the example given in the [neat README](https://github.com/thoughtbot/neat#getting-started)
* Use [World Wildlife Fund site as an example for wireframe](http://mediaqueri.es/wwf/)
* Add wireframe design for a reference to base our CSS off of
* Add base HTML to work from (this will change as we go work through neat)