[#19] Make logo look bigger on mobile screens

Fixes #19 

@StephenAbbott, how's this?

![screen shot 2014-11-26 at 13 11 26](https://cloud.githubusercontent.com/assets/282788/5201712/c41af436-756d-11e4-9736-97d960dee189.png)

The logo gets resized automatically by the responsive code, so I had to make the icon smaller in relation to the text. This allows it to fill up all of the available space.

Here's how it looks on desktops:

![screen shot 2014-11-26 at 13 12 32](https://cloud.githubusercontent.com/assets/282788/5201742/19fd7310-756e-11e4-9023-5279ca5dc510.png)