add improved btGeneric6DofSpring2Constraint, thanks to Puhr Gabor and Tamas Umenhoffer!

improved the new demo testbed (work-in-progress)
add basic Lua demo, import URDF test, STL import, obj import

src/BulletCollision/CollisionDispatch/btCollisionObject.h

@@ -92,11 +92,10 @@ protected:
 	int				m_internalType;
 
 	///users can point to their objects, m_userPointer is not used by Bullet, see setUserPointer/getUserPointer
-	union
-	{
-		void*			m_userObjectPointer;
-		int	m_userIndex;
-	};
+
+    void*			m_userObjectPointer;
+    
+    int	m_userIndex;
 
 	///time of impact calculation
 	btScalar		m_hitFraction;