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)


Don mentioned today that we can actually reference the translations directly without interpolating them. So this line could be changed to:

```
<%= link_to  t("navigation.contribute"), "#{locale_prefix}/contribute" %>
```