Add wrapper for AutoOrientImage()

[horgh]

I added a new public function to the Image type to call the MagickCore API function AutoOrientImage().

I noticed there is an open issue, #13, which asks for this too.

I've used it and added tests and it works well. Specifically why I wanted it is I found some JPGs I have to not display with the correct orientation when placed in HTML.

Please let me know if you would like me to change anything.

Thank you!