Summary:
This commit adds an HTML file for creating perceptual diffs (pdiff.html),
a reference screenshot of that HTML file (pdiff.png), and a quick script
for comparing that screenshot to a newly generated one (pdiff.js).
Also added a basic README.
Test Plan: make pdiff
Reviewers: alpert
Reviewed By: alpert
Differential Revision: http://phabricator.khanacademy.org/D6415
