Updating browser quickstart to align more with node quickstart.

Since @clarle updated the node quickstart, I updated the browser quickstart to be not only more in line with the node quickstart but also update the example to do an actual translation. The example is meant to be simple and easy to understand right away. I also included a link to a Gist with the full working example on the client.
`Intl.js` is a polyfill, so I don't think we should mention it in this first sentence. It's an implementation detail.

I don't think we need to discourage people from using the `Intl` and `IntlMessageFormat` APIs directly.