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