plumbing v2 `docker push`
Merge branch 'vbatts-v2_hacking' of https://github.com/vbatts/docker into vbatts-v2_hacking

Conflicts:
	graph/pull.go
Merge branch 'vbatts-v2_hacking' of https://github.com/vbatts/docker into vbatts-v2_hacking
vendored libtrust
Merge branch 'wip_provenance' of https://github.com/dmcgowan/docker into wip_provenance
Working impl. of push/pull - no sig. verification yet
Add signature verification