[css3-animations][css3-transitions] AnimationEvent and TransitionEvent should have constructors and schew init methods

I note that recent drafts of many specs—DOM4, HTML, Workers, WebSocket,
etc. specify events with constructors that take a dictionary of arguments,
and don’t specify an init*Event method. So instead of this:

var e = document.createEvent('TransitionEvent');
e.initEvent(type, false, true, 'background-color', 0.3);

This:

new TransitionEvent(type, {
    bubbles: false,
    cancelable: true,
    propertyName: 'background-color',
    elapsedTime: 0.3
  });

This is more convenient because it is a single expression and easier to
read because the arguments are effectively labeled by the hash properties.

You could look at the HashChangeEvent as a typical example of how the
constructor and dictionary are described in Web IDL. <
http://dev.w3.org/html5/spec/Overview.html#hashchangeevent>

It would be nice if CSS3 Animations and CSS3 Transitions adopted this style
for events.

Dominic