Basic perceptual diffing support for KaTeX

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