Use Broccoli and ES6

As per a discussion with @krisselden I've moved the project to a Grunt + Broccoli build. The dist generates AMD, CommonJS and a Globals build similar to rsvp.js.

* Travis needs to be enabled.
* I've pulled in the changes from HTMLBars, but I haven't bothered refactoring the lib into separate modules yet to keep the diff smaller.
* External assets are pulled in through bower.

`grunt dist` - Build dist (commonjs, amd, globals)
`grunt test` - Run tests in node
`grunt server` - Run tests in browser at <http://localhost:4200>