Add sCAL support to PNG reading and writing

It would be nice if the optional PNG - Physical Scale - sCAL chunk could be read and written.

http://www.libpng.org/pub/png/book/chapter11.html#png.ch11.div.9

For my use case, the pHYs chunk is not a good alternative. the pHYs x and y values are ints which lose necessary precision.

I have a patch (with junits) ready for your review if you are interested.

Thanks