Hack: stats.sh prints useful project stats for maintainers
Improve error message when refusing to remove image due to multiple repo tags
Remove reference to GH issue from error message
Fix registry pushing/tagging

* docker push host:port/namespace/repo wouldn't push multiple tags for
  the same image
* getImageList was unnecessarily complex returning a nested array of
  ImgData when a correctly ordered list of images was sufficient
* removed various bits of redundancy
Remove unneeded DependencyGraph
Further simplification
Fix indentation to match rest of file (tabs, not spaces)
Merge pull request #3114 from shykes/hack-stats

Hack: stats.sh prints useful project stats for maintainers
Prevent deletion of image if ANY container is depending on it; not just running containers
Merge branches '643-stale-nfs-handle', 'fix-registry-push-tags' and '1530-improve-error-message', remote-tracking branch 'docker/master' into dev