Make the async script play nice with JavaScript loaders

I ran into an issue using the zxcvbn-async.js script with the [head.js](http://headjs.com/) JavaScript loader. The async script would get run on the page after the window's load event, which means that at least in Internet Explorer, the main zxcvbn.js script would never get run.

Admittedly, this is a bit of an edge case. But now it'll be covered! :)