[FEATURE Beta] Benchmarks

This PR introduces benchmarks for Ember.js. Originally, it came out of
[Composable CP PR](https://github.com/emberjs/ember.js/pull/3696).

Benchmarks are powered by ES6 modules, grunt and
[benchmark.js](http://benchmarkjs.com/).

TODOs:
- [x] Benchmarks run in the browser
- [ ] Benchmarks run in the command line
- [ ] Grunt should pick up different version of Ember.js automatically
  (right now benchmarks run against two versions of Ember.js)
- [ ] Output results of the benchmarks
- [x] Add README
- [ ] Add more benchmark tests