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.
The way you have it, "size" is in the correct case as per [this](https://community.emc.com/servlet/JiveServlet/previewBody/3481-102-6-16527/Atmos%20Online%20Programmer's%20Guide%201.2.4A.pdf;jsessionid=360D5B1A4D136E51874AB885545A7FA4.node0). However, wouldn't it be better if this was a case insensitive comparison?