Fix race condition during initialization

[guillaumejenkins]

The current init logic first checks whether document.readyState === 'complete' before calling init(), otherwise it starts listening for the DOMContentLoaded event and sets init() as the event callback.

According to the DOMContentLoaded documentation, it can fire while document.readyState is still 'interactive'. So, the current logic has a race condition where if the DOMContentLoaded event has already fired before we can start listening to it, and the document.readyState isn't 'complete' yet, then init() will never be called. This was preventing the polyfill from working for the demo page (and other simple pages on the web like www.example.com) in some cases, such as in the FireFox browser.

The fix is to listen to the readystatechange event instead, because if document.readyState isn't 'complete' yet, then we are guaranteed that the final readystatechange event hasn't fired yet, so it's safe to wait for it.

Fixes #60

[tomayac]

LGTM, approval