bug#560: image-mode: add rotation

[jidanni]

image-mode needs the usual r and l to rotate 90 degrees, etc., else
one must rotate one's head to see vertical portraits.

The closest one finds is
  L runs the command image-dired-rotate-original-left
but that's not available in simple image mode.

Also it asks if one wants to overwrite the file each invocation. One
slip of the finger and your file gets overwritten.