Refactor to focus on WebGL Renderer

As discussed on https://github.com/threeDart/three.dart/issues/143 the goal here is to move other renderers to "examples" and focus on the WebGL renderer.

I also introduced a couple of interfaces for the materials hierarchy. 

Long term goal is to use mixins but since our Material uses optional arguments we'd have to introduce a new constructor which might be confusing.

@financeCoding @robsilv Please take this for a spin before I merge with master