Fix retina issue + updated iOS 6 icon.

Hi,

I have updated the iOS 6 icon to match the style of iOS 6 Safari's.

![arsafariactivity-ios6](https://cloud.githubusercontent.com/assets/28428/2564017/8dfa6abe-b892-11e3-82fe-bcc0fcc56eb1.png)

Also, updated the image loading logic to properly handle `@2x` images. I broke this on previous commit. My bad.

-R