Updated header

Made some DOM changes, so we'll need to watch those. Everything works as far as I can tell, but you guys probably have a lot more insight into what might break.

The menu slides in and out now. Theres some code to set the size of the timeline dynamically. Now that it's responsive, we don't need to let the user do any page scrolling.

As I said at the meeting, I opened up an issue for the icons we'd need here: #24.

Here's a screenshot:

![screen shot 2014-10-20 at 5 35 48 pm](https://cloud.githubusercontent.com/assets/6354033/4711399/41dc0214-58ba-11e4-9c93-129e40c3dfe3.png)