code: func (d *driver) URLFor(ctx context.Context, path string, options map[string]interface{}) (string, error) {
  	expiresTime := time.Now().UTC().Add(20 * time.Minute) // default expiration
  	expires, ok := options["expiry"]
  	if ok {
  		t, ok := expires.(time.Time)
  		if ok {
  			expiresTime = t
  		}
  	}
  	blobRef := d.client.GetContainerReference(d.container).GetBlobReference(path)
  	return blobRef.GetSASURI(azure.BlobSASOptions{
  		BlobServiceSASPermissions: azure.BlobServiceSASPermissions{
  			Read: true,
  		},
  		SASOptions: azure.SASOptions{
  			Expiry: expiresTime,
  		},
  	})
  }
docs: // URLFor returns a publicly accessible URL for the blob stored at given path
  // for specified duration by making use of Azure Storage Shared Access Signatures (SAS).
  // See https://msdn.microsoft.com/en-us/library/azure/ee395415.aspx for more info.