Add acorn as dependency (validator support)

We're switching away from Spidermonkey and using Acorn instead.

package.json

@@ -7,6 +7,7 @@
     "npm": ">= 1.1.x"
   },
   "dependencies": {
+    "acorn": "0.4.0",
     "clean-css": "0.10.x",
     "less": "1.3.x",
     "stylus": "0.32.x",