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/BulletDynamics/Dynamics/btDiscreteDynamicsWorld.cpp

@@ -317,6 +317,9 @@ void	btDiscreteDynamicsWorld::debugDrawWorld()
 			}
 		}
 	}
+    if (getDebugDrawer())
+        getDebugDrawer()->flushLines();
+
 }
 
 void	btDiscreteDynamicsWorld::clearForces()