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

README.md

@@ -0,0 +1,17 @@
+KaTeX
+=====
+
+Fast math!
+
+To run locally
+--------------
+    make setup
+    make serve
+
+* [Test page](localhost:7936/)
+* [Unit tests](localhost:7936/test/test.html)
+* [Perceptual diff](localhost:7936/test/pdiff.html)
+
+Perceptual diff
+---------------
+    make pdiff