Support LSD to detect quads

Use the LineSegmentDetector [1] algorithm instead of Canny+FindContours
to detect quads in the image.

The process is about x2 slower, but it significantly improves detection
rates.

The LSD algorithm replace Canny+FindContours when the compilation flag
EXPERIMENTAL_LSD is ON.

[1]
http://docs.opencv.org/trunk/modules/imgproc/doc/feature_detection.html#linesegmentdetector