Official image provenance pull flow

Adds initial support for provenance of official images.  Official build keys are pulled in from a static URL when an official image is requested.  If the registry version 2 is not available, the pull flow should still fallback to the version 1 registry.
Requiring golint to pass is probably the best way to achieve it.  That is the approach we are taking with libtrust and it makes the godoc much nicer.  http://godoc.org/github.com/docker/libtrust