[manifest] Address timeless' review comments (#383)

https://lists.w3.org/Archives/Public/public-webapps/2015AprJun/0794.html
You can view, comment on, or merge this pull request online at:

  https://github.com/w3c/manifest/pull/383

-- Commit Summary --

  * tidy
  * Installability signals
  * Navigation scope
  * Deep links
  * Display modes
  * display member
  * orientation member
  * start_url member
  * Content security policy of image objects
  * density member
  * sizes member
  * src member

-- File Changes --

    M index.html (133)

-- Patch Links --

https://github.com/w3c/manifest/pull/383.patch
https://github.com/w3c/manifest/pull/383.diff

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https://github.com/w3c/manifest/pull/383