Refacto with PID

It's quite a big refacto, so tell me if you prefer I make another repo.

#### Changes
- Control via pid
- Wait for drone stabilization before taking picture
- pid also determines the time before next control
- client.stop() if no face detected
- horizontal & vertical moves

#### Issues
The biggest problem is the detection of other random objects, which can make the drone crazy.

#### PID coefficients
Have be determined manually. I'm open to any other solution.