Add a 'docker fingerprint' command for signing images

The "should we do this" discussion starts in #2700 [1] (where it still
hasn't reached a consensus).  The goal here is to implement:

     $ docker fingerprint <namespace>/<name>[:<tag>]
     <namespace>/<name>:<tag>
     <metadata>
     <own tarsums> 

The tarsum should be the tarsum of the assembled image (squashing all
layers), not the tarsum of the tip layer.

The current implementation is a work in progress.  I'm still not clear
on how the whole server/job API is supposed to get wired together, and
have been basically pattern-matching off 'git grep -i history'.  I'm
posting the WIP branch because I won't have time to work on it for the
next few days, and wanted to open it up in case anyone else wanted to
chip in.

[1]: https://github.com/dotcloud/docker/issues/2700#issuecomment-42474265

https://github.com/dotcloud/docker/issues/2700#issuecomment-42474265