a-entity as camera no longer works, please use a-camera

the following no longer works with a-frame:

```<a-entity camera stereocam="eye:left;"></a-entity>```

you must use:

```<a-camera id="camera" stereocam="eye:left;"></a-camera>```

The examples also need to be updated to use this and modern a-frame 1.0.4+ to work in webxr