Pngquant

To run it on OS X you'll need `pngquant` executable in `Adobe Photoshop CC.app/Contents/MacOS/`.

I recommend multicore build from [pngquant.org](http://pngquant.org/):

     curl http://pngquant.org/pngquant.tar.bz2 | tar xz pngquant-openmp
     mv ./pngquant-openmp '/Applications/Adobe Photoshop CC/Adobe Photoshop CC.app/Contents/MacOS/pngquant'

This pull request changes how 8-bit PNG images are generated. Instead of generating palette with ImageMagick, [which is inefficient and buggy](http://www.imagemagick.org/discourse-server/viewtopic.php?t=23340) it uses ImageMagick to generate 32-bit PNG (to support padding and background features) and then converts image to PNG8 **with alpha** using `pngquant`.

Integration is done via streams, so no extra files are created.