Fix Lua binding of PhysicsWorld2D. Closes #250.

Source/Engine/LuaScript/pkgs/Urho2D/PhysicsWorld2D.pkg

@@ -11,7 +11,7 @@ class PhysicsWorld2D : public Component
     void SetWarmStarting(bool enable);
     void SetContinuousPhysics(bool enable);
     void SetSubStepping(bool enable);
-    void SetGravity(const Vector2& gravity);
+    void SetGravity(Vector2 gravity);
     void SetAutoClearForces(bool enable);
     void SetVelocityIterations(unsigned velocityIterations);
     void SetPositionIterations(unsigned positionIterations);
@@ -28,7 +28,7 @@ class PhysicsWorld2D : public Component
     bool GetContinuousPhysics() const;
     bool GetSubStepping() const;
     bool GetAutoClearForces() const;
-    const Vector2& GetGravity() const;
+    Vector2 GetGravity() const;
     unsigned GetVelocityIterations() const;
     unsigned GetPositionIterations() const;
 
@@ -42,7 +42,7 @@ class PhysicsWorld2D : public Component
     tolua_property__get_set bool continuousPhysics;
     tolua_property__get_set bool subStepping;
     tolua_property__get_set bool autoClearForces;
-    tolua_property__get_set Vector2& gravity;
+    tolua_property__get_set Vector2 gravity;
     tolua_property__get_set unsigned velocityIterations;
     tolua_property__get_set unsigned positionIterations;
 };