Optimize skin classifier

[felipelerena]

I'm looking at the _classify_skin method. (https://github.com/hhatto/nude.py/blob/master/nude.py#L283) I think there is a big waste of processing here since it returns rgb_classifier or norm_rgb_classifier or hsv_classifier or ycbcr_classifier. It seams useless to calculate the next classifier if the previous is True.