Initial release

The first experimental version of Three-rs library, includes:

• scene graph built with Froggy
• renderer with gfx-rs
• shading models: Flat, Gouraud, Phong
• light source types: Ambient, Directional, Hemisphere, Point
• basic directional light shadows
• Wavefront OBJ loader
• texture loader
• examples: lights, materials, obj, shapes, sprite, basic aviator