Ambient light source

In V0.7.5, the ambient light colour is taken from the SceneJS.Renderer#clearColor. The rationale was that the canvas colour represents the infinite scene "background".

However, we might want to show a model on a white background, in a scene with non-white (or even non-existent) ambient lighting. Solution is to provide an "ambient" LightSource type:

SceneJS.Lights({
      sources: [
         {
             type: "ambient",
             color: { r: .5, g: .5, b: .5 }
         }
      ]
});

When no "ambient" light is defined, the ambient light will default to the canvas colour, as it currently does.