Support configuration of Azure Cache-Control Headers

Currently cache-control headers cannot be easily set with azure blobs; because we assume that all user metadata has the same prefix. Cache-Control headers fall into the grey area of content vs user metadata, but we need a way to either allow for directly set headers, or to make an exception for the Cache-Control headers.

See: http://mail-archives.apache.org/mod_mbox/incubator-jclouds-user/201306.mbox/%3CCAJ7Hqj_aEpa1yh4BE%3DWz4ew%2BO5JnJhuPztgYDUE%2BY4-h%2BSpmNQ%40mail.gmail.com%3E