Use LSD instead of Canny+FindContours in FindQuads
Some optimization in FindQuads::mergePoints

Can be still improved.
Hide LSD begind EXPERIMENTAL_LSD flag

By turning ON this flag at compilation, chilitags uses
the LineSegmentDetector algorithm [1] to find quads in the
image.

[1]
http://docs.opencv.org/trunk/modules/imgproc/doc/feature_detection.html#linesegmentdetector
Eliminate the tag 682 from recognition

Tag 682 is a fully black marker. It can be quite easily falsely detected in dark zones.