support for Alpine package format (APK)

[majewsky]

• add generator for Alpine
  • check whether we should generate a /etc/apk/protected_paths.d file for config files in the package (i.e. all files until support packaging of applications #13 lands)
• test coverage for Alpine generator
  • figure out how to make the datahash computation more reproducible
• adjust documentation
• test that generated packages can be installed on an actual Alpine system

@AinRiver Please take care of the last item. My code is in the alpine branch in this repo. Feel free to fork and send PRs against that branch as necessary. I will merge into master once stuff is reasonably stable.

[ghost]

APKBUILD's are available at https://github.com/ainriver/aports, after some testing i'll send it for merge upstream.