Rewrite in pure Javascript

I opened ticket #5, which converts this into being a pure jQuery plugin, before seeing the commit in which you cleared things up to be less reliant on jQuery.

This branch removes any reliance on jQuery entirely, allowing it to be embedded in any page; the only dependency is the DOM, and specifically the `document.querySelectorAll` method — but since we're relying on media queries, I think we can also safely rely on that.

As a result, the call changes from $.appendAround('foo') to appendAround('foo'); I've updated the example HTML to reflect that too.