A few improvements

* Attempt to detect D3D9 or D3D11 ANGLE backend.  #10.  @toji - I'm open to better ideas.  See `getAngle` in the diff.
* Detect if context creation will fail due to a major performance problem, e.g., software renderer.  `failIfMajorPerformanceCaveat`
* Added tip for seeing draft extensions in Chrome and Firefox.
* Minor code cleanup and consistency improvements.

![image](https://f.cloud.github.com/assets/782098/1634628/17b376b6-579e-11e3-8a0d-e8b4a9346c36.png)

![image](https://f.cloud.github.com/assets/782098/1634631/2136d2be-579e-11e3-9797-fee5b1115cea.png)