Broccolification

Alright, this adds Broccoli and will replace the build process currently done via Grunt.

**TODO**

- [ ] compile sweet.js macros with https://github.com/sindresorhus/broccoli-sweetjs (currently getting an error which looks related to https://github.com/mozilla/sweet.js/issues/193)
- [ ] create `promises-spec.js` out of `node_modules/promises-aplus-tests/lib/tests/**/*.js` using https://github.com/kmiyashiro/broccoli-cjs-wrap and https://github.com/gingerhendrix/broccoli-browserify