Local install of grunt-cli to support `npm test`

Created by: tschaub

Leveraging npm scripts can let people get more done without global installs. To support npm install && npm test, the grunt-cli package can be added as a dev dependency. Even the grunt-cli devs have acknowledged the value in this.

I've only suggested a minor change here. If people are in favor of this direction, npm scripts could be added for common Grunt tasks. E.g. this would support npm run build && npm start (without a global Grunt install):

--- a/package.json
+++ b/package.json
@@ -14,7 +14,9 @@
   "homepage": "http://getbootstrap.com",
   "author": "Twitter, Inc.",
   "scripts": {
-    "test": "grunt test"
+    "test": "grunt test",
+    "build": "grunt dist",
+    "start": "grunt watch"
   },
   "style": "dist/css/bootstrap.css",
   "less": "less/bootstrap.less",