refact(es6): use es6 syntax.

* fixed #10, But iojs-2 not support Object.assign, so use object-assign
  shim instead for now.
* use es6 syntax.
* use eslint to instead jshint.

package.json

@@ -7,9 +7,9 @@
     "index.js"
   ],
   "scripts": {
-    "test": "mocha --harmony --check-leaks -R spec -t 5000 test/*.test.js",
+    "test": "eslint . && mocha --harmony --check-leaks -R spec -t 5000 test/*.test.js",
     "test-cov": "node --harmony node_modules/.bin/istanbul cover node_modules/.bin/_mocha -- --check-leaks -t 5000 test/*.test.js",
-    "jshint": "jshint .",
+    "lint": "eslint .",
     "autod": "autod -w --prefix '~'",
     "cnpm": "npm install --registry=https://registry.npm.taobao.org",
     "contributors": "contributors -f plain -o AUTHORS"
@@ -18,15 +18,15 @@
     "debug": "~2.2.0",
     "humanize-ms": "~1.0.1",
     "ini": "~1.3.4",
-    "merge-descriptors": "~1.0.0"
+    "object-assign": "~4.0.1"
   },
   "devDependencies": {
     "autod": "*",
     "beautify-benchmark": "0",
     "benchmark": "1",
     "contributors": "*",
+    "eslint": "~1.5.0",
     "istanbul-harmony": "*",
-    "jshint": "*",
     "koa": "1",
     "mm": "1",
     "mocha": "*",