Eliminated use of centered primitives.  Minor comment change.
Added 4_40_bolt_socket_loose module and fixed an error in
4_40_nut_socket.
Fixed the name of the Sharp photointerrupter module and associated
comment.
Added descriptive comments for some support modules.  Added
4_inch_gusseted_corner_brace module.
Added solarbotics_gearmotor_straight as an alias for
yellow_dc_gearmotor_straight.  Added solarbotics_gearmotor_angled
module.  Added nema_23_stepper_motor module.
Eliminated use of centered primitives.
Minor fixes to heights of cylinder interior cutouts.