Option to override palette for all frames

I've added an option to override palette for all frames. By passing the option `globalPalette` you can provide an `[r,g,b,...]` array, or `true` to use the palette of the first frame.

If it's set to `true`, only one worker is started at first. After the first frame, it's palette is stored, and the remaining workers are spawned.

This PR is based on my previous one https://github.com/jnordberg/gif.js/pull/26 with dithering.

You can see it in action here: http://depthy.me/