Refactor token verification to support x5c header

Docker-DCO-1.1-Signed-off-by: Josh Hawn <josh.hawn@docker.com> (github: jlhawn)
Refactored out usage of strconv.Quote()

Docker-DCO-1.1-Signed-off-by: Josh Hawn <josh.hawn@docker.com> (github: jlhawn)
Merge pull request #865 from stevvooe/next-generation

Integrate auth.AccessController into registry app
Merge pull request #862 from jlhawn/ng_auth_package

Refactor token verification to support x5c header
Ensure that unset Context.Name only allowed on base route

If Context.Name is not set, the acceess controller may allow an unintended
request through. By only allowing a request to proceed without a name on the
base route, we provide some protection if future bugs forget to set the context
properly.