Add 'apple touch icon url' site setting

This adds a site setting and default image for the Apple touch icon. This icon is used on all Apple touch devices for a few things, the most important being "add to home screen":

![ipad](https://f.cloud.github.com/assets/65323/620960/8d4e1a00-cef2-11e2-86fe-3885a73c9a58.png)