Switch Screenshotter data from JSON to YAML

Escaping TeX in JSON as query strings is a pain: you have to double all the
\\, you have to escape the & and the #, you can't easily include line breaks
for readability, and so on.  YAML solves most of these problems for most of
the situations.  Now each test case can be structured, while simple test
cases only consist of a line of verbatim TeX code, with no escaping.

The most troublesome items remaining are lines starting in { since in YAML
these would denote inline mapping types. We use block notation for these.

package.json

@@ -18,10 +18,11 @@
     "clean-css": "~2.2.15",
     "express": "~3.3.3",
     "jasmine-node": "2.0.0-beta4",
+    "js-yaml": "^3.3.1",
     "jshint": "^2.5.6",
     "less": "~1.7.5",
-    "selenium-webdriver": "^2.46.1",
     "nomnom": "^1.8.1",
+    "selenium-webdriver": "^2.46.1",
     "uglify-js": "~2.4.15"
   },
   "bin": "cli.js",