Add docker pull convenience commands

It would be nice to simplify our dockerhub interactions with scripts that automate the workflow. For example, if want to use an image from dockerhub we would do something like the following:

$docker pull $img
$ch-docker2tar $img /var/tmp/ch-tarballs/
$ch-tar2dir /var/tmp/ch-tarballs/$img.tar.gz /var/tmp/ch-images/