Update `apple-touch-icon-precomposed.png` and the docs related to the touch icons

### Documentation and other information

* From the Apple documentation https://developer.apple.com/library/ios/documentation/UserExperience/Conceptual/MobileHIG/IconMatrix.html:

  ![](https://cloud.githubusercontent.com/assets/1223565/4318621/2fbc70da-3f21-11e4-95f8-63330a30f336.png)

  (thanks @arthurvr)

* From: http://www.mobilexweb.com/blog/safari-ios8-iphone6-web-developers-designers:

  ![](https://cloud.githubusercontent.com/assets/1223565/4368461/6d9a6496-42ed-11e4-96d4-d2f51168748d.png)

    __edit:__ see https://github.com/h5bp/html5-boilerplate/pull/1599#issuecomment-56480775.

### Testing 

 The information from above needs to be tested and confirmed! (@mathiasbynens [updated his test page](https://github.com/h5bp/html5-boilerplate/pull/1599#issuecomment-56000419))

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Note: I'll update the commit as we go along and more information is confirmed.