added "retinafied" favicon

Updated the favicon.ico to include both 32x32 and 16x16 images so that it looks pretty on HiDPI displays (i.e. retina).

You can see the difference here:
![Screen Shot 2013-03-20 at 3 32 25 PM](https://f.cloud.github.com/assets/847027/282551/bf3e1276-9195-11e2-9cb1-33781b14d296.png)