The hokuyo model uses a ray sensor. The two parameters you're looking for are <min_angle> and <max_angle>. The reason you're only getting 180 degrees of readings is because the author of the tutorial set the min and max angle to -90 and 90 degrees (see the values here -1.57 and 1.57 radians). To get 360 degrees of range change the minimum angle to -pi radians, and the maximum angle to +pi radians.