JCLOUDS-654: Add object size to StorageMetadata

This allows callers to read the content length during container
listing.  Tested against: atmosonline, aws-s3, azureblob, filesystem,
and transient.  Intentionally not implemented for legacy swift
provider.
FWIW, I feel that developer names should be stripped from the codebase in favor of either "jclouds", "jclouds-user", or just "user". This applies to all instances in this PR.