Add VideoStats HUD element.

![fpsmeter](https://cloud.githubusercontent.com/assets/1821693/5321490/92b4a464-7c70-11e4-9aa3-526643e631b0.png)

Derived from mrdoob's stats.js.

Usage:
- Create VideoStats
- add to DOM using stats.domElement
- call stats.start(videoElement)
- Will display resolution and a framerate graph for that video element.

@samdutton @fippo @jiayl PTAL