Eat your vegetables: Broccolification

This commit replaces the Grunt tasks - which compile the ES6 router.js
modules into CommonJS, AMD and standalone Browser versions - with Broccoli.

A new build of router.js can now be created via `broccoli build dist`.
Automatic re-compilation is available via `broccoli serve`, while the
tests are executable via `http://localhost:4200/tests/index.html`.

Running the tests via the command line is possible via `grunt test`.
Remove unused Grunt dependencies

Grunt is now only used for executing the tests via the command line,
uploading the builds to S3 and invoking JSHint.