Get video material and screen overlay working.

Code pulled from Greg Beatty's (@gbeatty) Powder Tracks example application (http://cesiumjs.org/demos/powdertracks.html).

Here is a screen shot of the HelloWorld sample with a small Screen Overlay containing a sample video:

![image](https://cloud.githubusercontent.com/assets/1950836/3250659/6c39009e-f1a8-11e3-98b6-2e0a349f3998.png)

### CZML Used
```javascript
   {
       "id":"demo.webm",
       "availability":"2013-02-01T20:01:05Z/2013-02-01T20:03:03Z",
       "screenOverlay":{
          "width":256.0,
          "height":128.0,
          "show":true,
          "position":{
             "cartesian2":[
                20.0,40.0
             ]
          },
          "material":{
             "video":{
                "loop":true,
                "speed":1.0,
                "startTime":{
                   "date":"2013-02-01T20:01:05Z"
                },
                "video":"demo.webm"
             }
          }
       }
    }
```