Added missing API Get Repo Tags Json
Merge remote-tracking branch 'budhrg/go-docker-registry-0.9.0' into go-docker-registry-0.9.0
Updated Put Repo Tag as per docker-registry 0.9.0

* Updated create repo json to check 'docker' for docker version
  since user-agent contains 'docker' instead of 'docker_version'
Merge pull request #1 from rohanpn/go-docker-registry-0.9.0

Updated tags API(Added Get Repo Tags Json and Updated Put Repo Tag