Add grunt-cli to npm devDependencies.

According to grunt [docs](http://gruntjs.com/getting-started) you will need to install `grunt-cli` package to have `grunt` binary. 

Also the way you need to call `grunt connect` is `./node_modules/grunt-cli/bin/grunt connect`, in other case you'll need to add `-g` to `npm install`. I guess it could be described in README.