Responsive footer images

I noticed when browsing the 18F website that the footer image breaks at certain widths:

![screen shot 2014-05-06 at 7 38 14 am](https://cloud.githubusercontent.com/assets/212533/2888945/2a2ea646-d514-11e3-87ed-3ed62fb1e9f4.png)

Applying a max-width and max-height (rather than a height) fixes the problem:

```css
.footer img {
  max-width: 100%;
  max-height: 100px;
}
```

![screen shot 2014-05-06 at 7 42 56 am](https://cloud.githubusercontent.com/assets/212533/2888957/49562706-d514-11e3-9960-971ea83cf074.png)

At larger resolutions and on mobile, the footer appears the same as it did before:

![screen shot 2014-05-06 at 7 49 14 am](https://cloud.githubusercontent.com/assets/212533/2888977/954f890e-d514-11e3-998f-515d9b87d8db.png)

![screen shot 2014-05-06 at 7 49 32 am](https://cloud.githubusercontent.com/assets/212533/2888979/9968c442-d514-11e3-9b31-c6255fb1c7e4.png)


This was a two line change (see above), but the build process shows a larger diff. I hope this is useful and keep up the great work!