http://answers.opencv.org/question/40329/python-valueerror-too-many-values-to-unpack/
I bet you are using the current OpenCV's master branch: here the return statements have changed, see http://docs.opencv.org/trunk/modules/imgproc/doc/structural_analysis_and_shape_descriptors.html?highlight=cv2.findcontours#cv2.findContours.
Thus, change the correspinding line to:
_, contours, _ = cv2.findContours(skin_ycrcb, cv2.RETR_EXTERNAL, cv2.CHAIN_APPROX_SIMPLE)