Implement hardware interface

[adam-slucki]

Currently the steering and drive motor responds directly to messages sent to /cmd_vel topic. What's more, the node ignores what the values in the message actually represent.

To be able to use external ROS packages a hardware interface supporting our robot must be implemented.
Note: it must be done in C++, Python is not supported.

[adam-slucki]

Update: there's a working stub in hardware_interface branch.
Python object will be used for controlling pwm driver: https://learn.adafruit.com/16-channel-pwm-servo-driver?view=all

Python embedding in c++ part already works. Hardware Interface also works and correctly communicates with the ackermann controller.

Missing parts:

1. Ackermann controller - angular velocity isn't translated into steering angle #19
2. Converting requested velocity to PWM signal width