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

This freshens up @pornel 's contribution of a pngquant quantization pipeline for 8-bit pngs. It also adds an optional parameter to the `settings` object in `Generator.prototype.savePixmap` named `usePngquant` that enables the pngquant pipeline.

@pornel -- Thanks for the contribution! I made sure to incorporate your commit so that you get credit. :smile: However, I'm going to close #131 since I let that get waaaaay out of date and I can't update the branch that's in your repo. I really appreciate you bearing with us.

@iwehrman I've reviewed the code from @pornel but I've made a lot of changes. Could you give this a quick look?

This requires a recent Jenkins build of PS so that the pngquant binary is bundled.