Add support for SCRIPT tags

[nmoinvaz]

I was going through the polyfill used for AngularJS IE8 project (see the bottom of ie8.js). It seems to use an old version of your polyfill. However, I noticed it adds support for SCRIPT tags. If adding an event listener to a SCRIPT tag, it should trigger the listener when onreadystatechange is "loaded" or "complete".

https://msdn.microsoft.com/library/hh180173(v=vs.85).aspx