Implement more MediaPlayer code

* We now have working download of audio and video files, and the play()
method is called.
* Next step: implement play(), send the missing events to the
MediaPlayer object (when a video frame is available), and implement
paint (to draw the current frame on the web page).
HTML Audio working!

* Only tested with youtube videos :)
* The file is fully downloaded to memory before trying to decode it
* No seeking, no pausing, volume can't be set