Support Route 53 private hosted zones

- http://aws.amazon.com/releasenotes/3851444330669243
- http://docs.aws.amazon.com/Route53/latest/DeveloperGuide/hosted-zones-private.html
  - As of Nov 7, 2014 (JST), API document seems to be differ from actual
    implementation.
  - By looking aws-sdk-core-ruby's API manifest commit:
    - There's no "HostedZone>VPC", "HostedZone>VPCs" in ListHostedZoneResponse
    - There's no "HostedZone>VPCs", but "VPCs" exists outside of
      "HostedZone" in GetHostedZoneResponse
    - https://github.com/aws/aws-sdk-core-ruby/blob/3f176f9bf1c0e35307e8c75820a131dd5c4ade22/aws-sdk-core/apis/Route53.api.json#L1321-L1328