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:

- [x]  ~~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.~~ *Fixed, turned out to be an issue of case sensitivity and silent failure.*
- [x] Working CommonJS compilation.
- [x] Add tests for CommonJS on Node.
- [x] 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.
- [x] Make a pre-publish step to close https://github.com/tildeio/rsvp.js/issues/73
- [x] Add build instructions to README.md
- [x] Fix Travis CI (again)

Fixes #73, fixes #67, and fixes #49. Will fix #78 as soon as it's `npm publish`ed.