Add configs for web application manifest files

* Provide examples on how to set the correct MIME type, namely
  `application/manifest+json`.

* Configure Apache to serve these files:

   * compressed
   * with far-future expires headers (as recommended by the
     specification: http://w3c.github.io/manifest/#h_note_7)

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Specification:

   * http://w3c.github.io/manifest/

Support coming soon to:

   * Chrome: https://code.google.com/p/chromium/issues/detail?id=366145
   * Firefox: https://bugzilla.mozilla.org/show_bug.cgi?id=997779

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Ref: http://w3c.github.io/manifest/
     h5bp/server-configs-apache#29

Close: h5bp/server-configs-apache#29
Improve inline comments
Remove explanatory comments from .htaccess fixture