There's a more advanced version here: http://www.usingenglish.com/members/text-analysis/
The Help page explains how the calculations are made.
The routines and algorithms are publicly available, the code to implement them was written in-house by Adam King, the webmaster here, together with an additional tool provided by Alex Russell, a teacher and games writer. I've never cited it, but I would follow your style manual's guides for a webpage.
They have been tested by a number of people for accuracy, but are provided on an as-is basis.