Improved SphereGeometry UV Map at Poles

Modified `SphereGeometry` UVs at the poles so there is less distortion.

Before

![old sphere uv map](https://f.cloud.github.com/assets/1000017/1535244/f134c402-4ca0-11e3-8490-85e4936ee1f0.png)

After

![new sphere uv map](https://f.cloud.github.com/assets/1000017/1535245/fb0ec54a-4ca0-11e3-9456-cbddc744ab17.png)