update jQuery plugin to be compatible with ES/CommonJS modules

[WickyNilliams]

After this change, the following should be possible:

import $ from "jquery";
import Headroom from "headroom.js"
import registerJQueryHeadroom from "headroom.js/dist/jQuery.headroom.js";

registerJQueryHeadroom($, Headroom);

$("header").headroom();

[WickyNilliams]

Added a tests for jquery, and refactored the test suite so we can run the exact same set of tests for vanilla and jquery solutions.

[WickyNilliams]

I'm considering bundling Headroom with the jquery plugin itself, so the above code sample would be reduced to a much less boilerplate-y:

import $ from "jquery";
import registerJQueryHeadroom from "headroom.js/dist/jQuery.headroom.js";

registerJQueryHeadroom($);
$("header").headroom();