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

test/pdiff.js

@@ -0,0 +1,37 @@
+var fs = require('fs'),
+    gm = require('gm'),
+    path = require('path');
+
+var original = path.join(__dirname, 'pdiff.png'),
+    modified = path.join(__dirname, '../build/pdiff.png'),
+    difference = path.join(__dirname, '../build/DIFF.png');
+
+var colors = {
+    reset: "\033[0m",
+    red: "\033[31m",
+    green: "\033[32m"
+};
+
+function log(message, color) {
+    console.log(color + message + colors.reset);
+}
+
+gm.compare(original, modified, /* tolerance */ 0, function(err, isEqual) {
+    if (err) {
+        console.error(err);
+    } else if (isEqual) {
+        log("No perceptible differences.", colors.green);
+        // Remove any previously generated difference images
+        fs.unlink(difference, function() {});
+    } else {
+        log("Perceptible difference detected! See build/DIFF.png", colors.red);
+        // Generate new difference image
+        gm.compare(original, modified, {
+            /* output */ file: difference
+        }, function(err) {
+            if (err) {
+                console.error(err);
+            }
+        });
+    }
+});