Audio Support

Just some basic changes to make VJS work a little better out of the box for audio support.
Hm... maybe I'm over thinking things.

Are audio and video vjs players separate? I.e., can you have something like
```js
var player = videojs(document.createElement('audio'));
player.src({src: 'http://example.com/audio.mp3', type: 'audio/mp3'}); // or whatever the mimetype is

//now switch to a video
player.src({ src: 'http://example.com/video.mp4', type: 'video/mp4'});
```
Do we want to allow that switch? Also, if we allow that switch, the inactivity timeout will be disabled for the video-player it was disabled for the audio here.