Add jshint

* add npm package
* add npm pretest script

package.json

@@ -4,6 +4,7 @@
   "description": "Reimplementation of CSS layout using pure JavaScript",
   "main": "src/Layout.js",
   "scripts": {
+    "pretest": "./node_modules/jshint/bin/jshint src",
     "test": "./node_modules/karma/bin/karma start ./karma.conf.js --single-run"
   },
   "repository": {
@@ -18,6 +19,7 @@
   "homepage": "https://github.com/facebook/css-layout",
   "devDependencies": {
     "jasmine-core": "^2.2.0",
+    "jshint": "^2.6.0",
     "karma": "^0.12.31",
     "karma-chrome-launcher": "^0.1.7",
     "karma-jasmine": "^0.3.5"