Added speed control to arbotix_terminal and arbotix_python

I'm hoping to use arbotix_ros instead of dynamixel_conrollers in "Volume 2" of RBX but I need speed control to make this work.  I've tested the changes with both a USB2Dynamixel and an Arbotix using both AX12 and RX24F servos.  It holds up well with real time face tracking with a pan-and-tilt head.

The first commit places a lower limit of 1 instead of 0 on the value of servo speed so that users don't actually move the servo at max speed when they actually meant to stop it.

The second commit adds a 'set spd' option to arbotix_terminal.

The third commit adds a set_speed service to each servo_controller.