Add saving support for YA8 and YA16 images
Add saving support for YA8 and YA16 images

It seems to work, but it still does not specify ExtraSamples
tag to state that stored data is in fact an alpha channel.
See bug #56 .

clinty/Juicy.Pixels,Twinside/Juicy.Pixels,Chobbes/Juicy.Pixels,Twinside/Juicy.Pixels,Chobbes/Juicy.Pixels,clinty/Juicy.Pixels,Twinside/Juicy.Pixels