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
Currently there is one COMMENT per DockerFile  not per layer.  I am not sure how you could put multiple in.   I am not sure what the parser does if you put multipe COMMENT lines after each run.  I don't currently have this setup, but I could try that out to see if you get a comment per layer.

Base images are now all labeled based on the "imported from -" line if you imported them using cat TARBALL | docker import -.  My patch allows you to override this with the -m option.
If you use docker commit -m MESSAGE, then they will have comment = MESSAGE.

Also.