Renamed gruntfile.js to Gruntfile.js

The Gruntfile should be named Gruntfile.js instead of gruntfile.js (for Grunt 0.4). Libraries that look for "Gruntfile.js" (e.g. [Heroku buildpack: Node.js with grunt support](https://github.com/circuithub/heroku-buildpack-nodejs-grunt)) would not work otherwise.