Browser quickstart v2

This updates the browser quickstart and follows the suggestions by @ericf  in terms of organization and content. 

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I think we need to step back and figure out what the goals are for getting people to use Intl in the browser.

*  I would start with examples around Number and Date formatting (which are built into the modern browsers). [done]

* From there we can talk about what browsers support window.Intl, and then how to conditionally load the Intl.js polyfill in the browsers which do not support it. [done]

* Now that the basics of using Intl are established we could then move on to more complex messages, and introduce intl-messageformat, give some examples, touch on the feature highlights, link off to its main docs page, etc. This would also require showing people how to load this onto their page. [done] 

* Finally, I think we'd then provide people with a menu on where to go next, and that would be links to each of the template engine pages. [There are links at the bottom of the article to the future locations, and we can add the menu once that aspect of the site has been designed]

Let's just tell them the main option here. This makes it look more complex than conditionally loading the polyfill, when in fact it's simpler.

Also shouldn't this be above the first time we use the lib?