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/LinearMath/btIDebugDraw.h

@@ -438,6 +438,11 @@ class	btIDebugDraw
 		drawLine(transform*pt0,transform*pt1,color);
 		drawLine(transform*pt2,transform*pt3,color);
 	}
+    
+    virtual void flushLines()
+    {
+        
+    }
 };