Add GMTUsage class to track x-gmt-usage header data
Add policy parameter support to copy_key().

- Similar to other API which add a key or a bucket to the S3 server,
  Allow the user to override the default canned-acl which is applied
  to the new key. This default policy override is only used if the
  preserve_acl parameter is False.
Added Cloudian Virtual Bucket Support
Add Cloudian group canned acl names.