support for nested containers

 - this allows us to support casks for things like "dmg in zip" "zip in
   tar", etc, etc
 - a nested container can be any format that homebrew-cask supports
 - the existing container support is now referred to as a Cask's
   "primary container"
 - use the `nested_container` artifact type to indicate the relative
   path to a nested container that you wanted extracted
 - multiple layers of nesting should work with multiple nested_container
   directives (though this is untested as yet)
 - add SpeedDownload as the first cask to use this feature; refs #602
Merge pull request #1232 from phinze/dev/nested-containers

support for nested containers