added path from go image to opencv image and back.

Possible fix for #11

Method signatures are open for discussion.

The safest way for converting a go Image to an opencv IplImage (and vice-versa) I found, was to really convert and copy each pixel.

I did not find any way to share the same image buffer. This would also be very dangerous imho.
Thanks for doing this!  It might be worth benchmarking these methods against using the `Pix` array directly.  The code will be more verbose but this is likely to be performance sensitive.