Added initialization with flashcode inspired in visp_auto_tracker maintained by Fabien Spindler.

I'm kind of new in visp. I added initialization with flashcode using visp_auto_tracker detectors. I copied the detectors folder but maybe its better to somehow use visp_auto_tracker's one as a dependency? My only concern is the fiability of the estimation of the position of the qrcode frame based only in the four points, maybe I'm missing something.

Best,
Mauricio Garcia.