HTML5 video implementation

Hi Bernhard,

This is an implementation proposal for videos in StageXL.

It include :
* VideoData Class => the loaded html5 video in warped into this object
* Video Class => DisplayObject extension containing a videoData and all the control methods
* ChromaKeyFilter class => which is simple chroma key WebGL filter


Here is a very simple code sample of the implementation:
```dart
var rs = new ResourceManager()

void main() {
  rs..addVideoData('video', 'video.mp4');

  rs.load().then((_) {
    var videoData = _resourceManager.getVideoData("video");
    var video = new Video(videoData, autoplay: false, loop: true, frameRate: 24);

    stage.addChild(video);

    video.play();
  })
}
```

For more advanced testing I've created a little test page,
which can be found here https://github.com/sebgeelen/StageXL_video_demo .
I'll also try to put a test page online tomorrow, with a few examples.


You can find the detailed commits on this non squashed branch
(https://github.com/sebgeelen/StageXL/tree/video-element-implementation)


Seb