Add a retina favicon

Using the Illustrator files that @kytrinyx provided me, I added a retina `favicon.ico` (32x32). I also added a 180x180 (as per iOS 8 specs) Apple touch icon.

It looks gorgeous on the retina screen :smile_cat: 

Before:
![screen shot 2014-11-26 at 10 20 26](https://cloud.githubusercontent.com/assets/3890250/5198806/f65dcdfa-7555-11e4-912b-8814a3daaa0c.png)

After:
![screen shot 2014-11-26 at 10 20 31](https://cloud.githubusercontent.com/assets/3890250/5198808/fa13ccf6-7555-11e4-8343-82e1af679c0b.png)