Added stubs for CC3Texture and minor fixes to CC3Vector and CC3BoundingBox

* CC3Vector: fixed up indentation
* CC3BoundingBox: fixed bug for CC3VectorNull
Adding stub code to layout structure of library
Preparing for setup for feature-driven dev

* Added Cocos3DShowcase app
* Added stub classes to layout design
* Removed submodule cocos2d-xna to replaced with forked repo