useAlphaFromDiffuseTexture option for standard material

enable this option on a standard material make the shader algorithm use the alpha channel on the diffuse texture.

This avoid aliasing effect on transparent textures. See example [demo](http://arthur-brongniart.fr/Stockage/BABYLON%20useAlphaFromDiffuseTexture/demo.html)

![demo](https://cloud.githubusercontent.com/assets/1659820/2957207/ee503760-da9b-11e3-9861-44f33519885d.png)


discussion here http://www.html5gamedevs.com/topic/6304-alpha-channel-on-a-texture/