DOM event handling

[driverdan]

Add proper DOM event hander (addEventListener) and full event support. See http://dvcs.w3.org/hg/progress/raw-file/tip/Overview.html.

[driverdan]

Now supports

• addEventListener
• removeEventListener
• dispatchEvent

But is missing support for a few events such as abort.

[aamster]

@driverdan I have code like this:

xhr.addEventListener('load', e => {
  ....
});

but e is undefined. Is it expected?