Switch to ES6 Polyfill

I think this project should switch over to @jakearchibald's [es6-promise](https://github.com/jakearchibald/es6-promise) polyfill. Its becoming the community de facto standard. It also does an existing `defined Promise` check so it does not override the native implementation in Chrome.

Second, (and maybe conflating an additional PR into this one), it'd be nice to list `es6-promise` as a bower dependency and default `main` to the sans promise file. Its a little more convenient to dedup the `es6-promise` lib in a production bundle output. 

/cc @guybedford