Began move from Rake-based build to Grunt

Worked with @stefanpenner today on this.

All relevant Rakefile behavior is duplicated in the Gruntfile, and building is now as easy as `npm install rsvp.js && grunt build`.

Outstanding issues:

* Travis CI fails to find the Mocha tests. It works fine locally. Will need some debugging from someone more familiar with Travis, Browserify (for [promises-tests](https://github.com/promises-aplus/promises-tests)), and Mocha.
* Need testing in CommonJS/Node.
* Repo-specific tasks should be separated from the rest to identify which tasks can be universal between Ember microlibraries, with the goal of creating an `ember-dev` package for shared tasks & configuration.