Make compatible with Firefox 3.6

I've made some changes to the polyfill to make it compatible with Firefox 3.6.

* `getComputedStyle` needs second argument, set to null as per default value - https://developer.mozilla.org/en/docs/Web/API/window.getComputedStyle
* `addEventListener` needs third argument (`useCapture`), set to false as per default value - https://developer.mozilla.org/en-US/docs/Web/API/EventTarget.addEventListener

Regarding `addEventListener` - I haven't used method `addEvent` since IE doesn't accept third argument for `attachEvent` and I wanted to be sure there won't be any quirks if I pass non-existent argument (since it's browser core method). If you have better idea on how to approach this, please say :)