add MultiDofDemo (Featherstone 3DOF spherical joint)

minor prettify of BasicDemo,RagdollDemo.
require 'multiDof' argument in btMultiBody.h (not default=false)

src/BulletDynamics/Featherstone/btMultiBody.h

@@ -50,7 +50,7 @@ public:
               const btVector3 &inertia,    // inertia of base, in base frame; assumed diagonal
               bool fixedBase,           // whether the base is fixed (true) or can move (false)
               bool canSleep,
-			  bool multiDof = false
+			  bool multiDof
 			  );
 
     ~btMultiBody();