Use waypoints to animate global nav

* Add `waypoints` to project.
* Use `translate` to create fade-in effect in CSS.

https://trello.com/c/XJHlxDcO

![middleman-fixed-globalnav-waypoints](https://cloud.githubusercontent.com/assets/816402/4983915/e76d15aa-691e-11e4-9345-252e7d7f70b4.gif)


Maybe we could just change the name of the class to `.doc-item-active` so we don't have to chain all of these classes? We would need to change the name of the class in the helper method in `config.rb` as well.