ros_pololu_servo changes for demo

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