Update brightness & saturation indices calculation
Update brightness & saturation indices calculation

As per the research paper followed for building this algorithm (see:
http://hplabs.hp.com/techreports/2005/HPL-2005-14.pdf), saturation
index and brightness index are defined as:

Saturation Index: "To implement this, we compute the color saturation
and subtracted the average color saturation of the blurry areas from
that of the sharp areas."

Brightness Index: "To implement this, we compute the average
brightness difference between the sharp and blurry areas."

These changes fix how the brightness and saturation indices are
computed to match the paper.

tokenrove/blur-detection,tokenrove/blur-detection,tokenrove/blur-detection,tokenrove/blur-detection