Rename Apple touch icon & add <link> in index.html

Until now we recommended to our users to include an image named
`apple-touch-icon-precomposed.png` in the root of their website.

Since the iOS 8 release however, Safari will no longer request¹
the `/apple-touch-icon-precomposed.png` by default, and instead,
it will make the following requests¹:

    /apple-touch-icon-152x152-precomposed.png
    /apple-touch-icon-152x152.png
    /apple-touch-icon.png

In order to fix this issue we decided¹ to rename
`apple-touch-icon-precomposed.png` to `apple-touch-icon.png`,
and include a `<link>` element in the `index.html`:

    <link rel="apple-touch-icon" href="apple-touch-icon.png">

Notes:

  * Just renaming `apple-touch-icon-precomposed.png` to
    `apple-touch-icon.png` would have also worked², even with
    older version of iOS. Unfortunately, it wouldn't have
    represented a good solution as Apple may change again what
    is requested by default, plus, it would have generated quite
    a few 404s³.

  * We decided to rename the touch icon because most iOS users⁴
    are on iOS 7+, so the `precomposed` keyword no longer brings
    much benefit (as of iOS 7, no special effects are applied to
    touch icons).

  * Adding `<link rel="apple-touch-icon" ...>` in `index.html`
    will also allow⁵ (some of) the default Android 2.2+ browsers
    to use the touch icon.

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¹ h5bp/html5-boilerplate#1615

² https://mathiasbynens.be/notes/touch-icons
  https://developer.apple.com/library/ios/documentation/AppleApplications/Reference/SafariWebContent/ConfiguringWebApplications/ConfiguringWebApplications.html

³ https://code.google.com/p/chromium/issues/detail?id=259681
  h5bp/html5-boilerplate#1615

⁴ According to Apple (as measured by the App Store on October 13, 2014):

    * 5%  iOS < 7
    * 47% iOS 7
    * 48% iOS 8

  From: https://developer.apple.com/support/appstore/
        https://cloud.githubusercontent.com/assets/1223565/4720999/2aa8ba26-5935-11e4-9492-92ff6a50ad7d.png

⁵ http://www.ravelrumba.com/blog/android-apple-touch-icon/

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Close h5bp/html5-boilerplate#1622