Refactor package as ES6 module

This major refactor does the following:

* Refactor src/ as ES6 module.
* Use [`React.PropTypes`](https://facebook.github.io/react/docs/reusable-components.html).
* Add `intlTime()` method to Mixin.
* Revamp the build/packaging process to output Bundle and CommonJS versions.
* Update `package.json` and `bower.json`.
* Update usage of globals in example.
* Update dependencies on other packages in our Intl suite.
Yeah I didn't even look at what it was doing, I just deleted the entire contents of the `Gruntfile.js` and started fresh :smile: