Robust line tracking

Now the line tracking algorithm is made more robust and complete as can be seen in the video below.
After the calibration, the Rover first searches the tape, when it has found some tape it stops, makes a sound and then start driving. After the tape stops, it again makes a sound and then displays the length of the line (in mm) on the lcd-screen.
The controller still needs some tuning for a smoother tracking, but it is already a little bit robust for line cracks and other variations.
The main goal now is to tune the controller correctly and also let it work for different tapes and surfaces (for example a black surface and white tape).