JCLOUDS-161: large blob support for Azure

Large blob support for AzureClient; the next step of this is to
support PutOptions.multipart and digest a blob into 4M parts. This
just implements the Azure interaction.
Cleanup of Azureblob code; as per pull request review. Additional items
not covered here are assigned specific bug ids.
Follow-up to Azureblob cleanup

See discussion at https://github.com/jclouds/jclouds/pull/66