Save "COMMENT" field in Dockerfile into image content.

This will allow a user to save COMMENT data into an image, which
can later be retrieved using:

docker inspect IMAGEID

Copied the test example and some description data from Erik Hollensbe
<github@hollensbe.org>

Add support to allow docker import to populate Comment field

This patch also adds an option to docker import -m  Which will allow
base image builder to populate docker images with more comments then
just imported from FOOBAR.  This matches the docker commit -m functionality.

Docker-DCO-1.1-Signed-off-by: Dan Walsh <dwalsh@redhat.com> (github: rhatdan)