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

package.json

@@ -1,10 +1,16 @@
 {
     "name": "katex",
     "version": "0.0.1",
+    "description": "Fast math!",
+    "repository": {
+        "type": "git",
+        "url": "git://github.com/Khan/KaTeX.git"
+    },
     "devDependencies": {
         "browserify": "~2.29.1",
         "express": "~3.3.3",
         "less": "~1.4.2",
-        "through": "~2.3.4"
+        "through": "~2.3.4",
+        "gm": "~1.14.2"
     }
 }