Added parameters to enable or disable a type of texture on all st...

...andard materials.

This pull request allow us to enable or disable a type of texture on all standard materials. The method texturesEnabled in Babylon.Scene.js is usefull to disable all type of textures but in some case it's usefull to disable only some types.

It's simply to use, for example if you can disable only bump and specular textures, you have to write:
Babylon.StandardMaterial.bumpTextureEnabled = false;
Babylon.StandardMaterial.specularTextureEnabled = false;

By default, all types are set to true because it's the current behaviour.