allow passing type instead of guessing from src

this allows to pass videos like this

```js
videos = [
  {
    src : [
      {src: 'http://vjs.zencdn.net/v/oceans.wtf', type: 'video/whatever'},
      'http://vjs.zencdn.net/v/oceans.webm'
    ],
    poster : 'http://www.videojs.com/img/poster.jpg',
    title : 'Ocean'
  }
];
```