New service messages for dynamixel MX controller.
Modified CMakeLists to include new service messages.
Added current (amps) field to JointState msg.
Added new constants and register set methods for MX series.
Created joint_controller_mx

This class gives access to MX Dynamixel functionality, like current
sensing, real torque mode, and actual velocity enforcement.
Changed zero speed for joint_states by 1 bit.
Ensured proper limits on speed, position and torque.
Added current (amps) reporting for MX series.
Added constants specific to MX series.
Dynamixel IO with logging on all serial comm.

This version of io also allows for servos to be removed from the
background motor state polling.  This is primarily to allow dynamixels
to be put into position mode, then left alone for the necessary 100ms
or so before they regain their senses.
Now enables state polling after node is initialized.
Added stall_current to parameters.
Added stall current values to CONST.
Removed logging on serial communication.
Merge branch 'mx_controller' into SampleReturn