Implement BulkDelete to delete many objects at once - fixes #9

See: http://docs.openstack.org/trunk/openstack-object-storage/admin/content/object-storage-bulk-delete.html
Add Yamamoto, Hirotaka to contributors
Make BulkDelete return Forbidden if not supported by server

  * Remove Container from RequestOpts as on old servers that caused it
    to delete the container!
  * Add ErrorMap to RequestOpts
  * Add Forbidden error (403) so users can check for equality
  * Change BulkDeleteResult to return a swift.Headers not http.Headers