Fixed visualization by choosing the color appropriate to the detection

Fixed visualization by choosing the color appropriate to the detection
score.
Previously the example showed all detections with the same color
disregarding the confidence. This led to the impression that the object
detection did not work at all because there are many detections with low
confidences.

PR to master was
https://github.com/Itseez/opencv/pull/320
Merge pull request #330 from sromberg:2.4