Finish hamburger icon

I didn't worry about doing JS shenanigans. Eventually, it would be nice so that:
```
Open menu -> darker hamburger
Closed menu -> lighter hamburger
```
However, for faster deployment, I opted for
```
Hamburger not being pressed -> lighter hamburger
- being hovered -> medium hamburger
- being pressed -> darker hamburger
```
Moral of the story: it doesn't handle states.
![hamburger](https://cloud.githubusercontent.com/assets/7784737/5501183/80550378-870a-11e4-8c10-406c768484bc.gif)