tools: update ESLint, fix unused vars bug

Update ESLint to 3.0.0. This includes an enhancement to `no-unused-vars`
such that it finds a few instances in our code base that it did not find
previously (fixed in previous commits readying this for landing).

PR-URL: https://github.com/nodejs/node/pull/7601
Reviewed-By: Michaël Zasso <mic.besace@gmail.com>
Reviewed-By: Roman Reiss <me@silverwind.io>

tools/eslint/node_modules/string_decoder/package.json

@@ -43,7 +43,7 @@
   "directories": {},
   "dist": {
     "shasum": "62e203bc41766c6c28c9fc84301dab1c5310fa94",
-    "tarball": "http://registry.npmjs.org/string_decoder/-/string_decoder-0.10.31.tgz"
+    "tarball": "https://registry.npmjs.org/string_decoder/-/string_decoder-0.10.31.tgz"
   },
   "gitHead": "d46d4fd87cf1d06e031c23f1ba170ca7d4ade9a0",
   "homepage": "https://github.com/rvagg/string_decoder",