add srcObject and mozSrcObject attributes

In previous versions of firefox in chrome (pre window.URL) this is how you attached MediaStreams to a video element.

```js
el.srcObject = stream; // chrome and opera
el.mozSrcObject = stream; // firefox
```

To support those browsers I am proposing that these attributes be added so you can construct a video element like so:

```js
// chrome
React.DOM.video({
  srcObject: stream
});

// firefox
React.DOM.video({
  mozSrcObject: stream
});
```

(CLA signed)