ros_pololu_servo changes for demo

[jdddog]

I've made a couple of changes for the demo:

• Can control motor's via name rather than id
• Msg names are now similar to dynamixel_msgs stack
• Can calibrate radians to pwm mapping in yaml file (need to be able set pololu pwm to change the pwm range tho)
• Can specify init value in degrees in yaml file (which becomes zero) and the min / max angles from zero that the motor can move
• example launch files
• can set parameters via ros parameters
• Refactored library so its split into classes