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 #862 from jlhawn/ng_auth_package

Refactor token verification to support x5c header