Added Joint:setUserData and Joint:getUserData (resolves issue #894.)

src/modules/physics/box2d/wrap_FrictionJoint.cpp

@@ -78,6 +78,8 @@ static const luaL_Reg functions[] =
 	{ "getReactionForce", w_Joint_getReactionForce },
 	{ "getReactionTorque", w_Joint_getReactionTorque },
 	{ "getCollideConnected", w_Joint_getCollideConnected },
+	{ "setUserData", w_Joint_setUserData },
+	{ "getUserData", w_Joint_getUserData },
 	{ "destroy", w_Joint_destroy },
 	{ 0, 0 }
 };