Drop SweetJS in favour of ES6

I <3 SweetJS, but after seeing a bit more ES6 in the wild, and getting to grips with the concepts (hat tip to recent videos by @guybedford and @jakearchibald), I decided to use it wholeheartedly. Final x-gif.js size increased very slightly (15kb to 16kb gzipped), but I think the code itself is a lot better.

Loving the ES6 module syntax and es6 promises are super simple but super awesome.

Beware, totes broke in Safari 8 (https://github.com/Polymer/platform/issues/66) but works fine in Chrome and FF so ¯\_(ツ)_/¯