Integrated with pngquant

Assumes Photoshop CC includes pngquant 2.x executable from http://pngquant.org
add usePngquant parameter to savePixmap, clean up pngquant integration
Merge pull request #236 from adobe-photoshop/jrb/pornel-pngquant

add optional pngquant quantization of 8-bit pngs, add option to savePixmap to enable