SpotLight.angle and SpotlightHelper cone size bug fixes

1. Fixed SpotlightHelper cone size, which was incorrectly calculated. Now the helper's cone matches the spotlight's extent.

2. There has apparently been confusion as to the meaning of `SpotLight.angle`. The definition in the docs is correct: "Maximum extent of the spotlight, in radians, from its direction." This means that the maximum reasonable value for `angle` is `Math.PI/2`. Larger values were used in the examples. I fixed the examples as needed.

3. Spotlights throw light backwards (see image below). I did not fix this in the shaders; it is not an issue if `SpotLight.angle <= Math.PI/2`.

4. I changed the default `SpotLight.angle` to `Math.PI/3`, so it casts light more like a spotlight would. I would prefer to have set the default to `Math.PI/4`, which may have been the original intention, but decided to use `Math.PI/3`, a less drastic change.

![Screen Shot 2013-04-09 at 8 56 52 PM](https://f.cloud.github.com/assets/1000017/360084/4b77526a-a17d-11e2-9c2c-9b8d6e5dbf85.png)