Add Amazon S3 as an alternative publisher to GitHub pages

[raphaelcohn]

Add Amazon S3 as an alternative (or in addition to) publisher to GitHub pages.
Whilst this potentially is a more costly solution, it does allow:-

• specifying MIME types correctly for files
• using server redirects for files that are genuine externally hosted files (eg RPM-GPG-KEY-*, Release.key which reside on a HTTPS accessible keyserver)
• using server redirects for internal symlinks, eg repomd.xml.key, which should redirect to ../RPM-GPG-KEY-*
  • this would require using readlink or realpath
  • it would also require being able to recognise a symlink did not point outside of the repository
• storing additional useful data in HTTP headers, including Content-MD5, Date and ETag, so enabling proxies and CDNs