Use FontAwesome icons instead of images.

Using FontAwesome icons makes the logos more customizable and responsive. They have a slight darken affect on hover, and link to the downloads section. To change the color of them, use the class `.os` in `shoes.css` (line 28). I made them responsive and change size as the window gets smaller. This should fix [issue #35](https://github.com/shoes/shoesrb.com/issues/35).