Dithering support

I have merged the work of @PAEz, mentioned in https://github.com/jnordberg/gif.js/issues/12 , with some small changes (pixel format and clamping). It works pretty darn well!

You can pass `dither` in options with dither method's name, like `Stucki`. You can suffix it with `-serpentine` to enable serpentine scanning.

I've changed lookupRGB with findClosestRGB because I'm working on a global palette PR. But it also makes sense here... 

Hope both of you like it :)