Update `apple-touch-icon-precomposed.png` and docs

 * Update the documentation regarding the Apple touch icons, by
   reflecting the recent changes brought by the release of iOS 8 and the
   new iPhone 6 and 6 Plus.

   Note: The changes do not follow all Apple's iOS Human Interface
   Guidelines¹, as in practice, the images used by the different devices
   are different² from the ones specified in the Apple documentation.

 * Update the `apple-touch-icon-precomposed.png` to be `180×180px` (this
   is now the largest image size used by any Apple device running iOS).

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¹ https://developer.apple.com/library/ios/documentation/UserExperience/Conceptual/MobileHIG/IconMatrix.html
² h5bp/html5-boilerplate#1599

Close h5bp/html5-boilerplate#1599.