Use grunt-plugin-list app to serve grunt plugins (fixes #19)

Retrieve the list of grunt plugins as a single JSON file from http://grunt-plugin-list.herokuapp.com instead of getting every plugin's package.json separately from the npm registry. This speeds up the time to load the plugin information considerably (fixes #19).

The source code for the heroku app is available at [manuel-woelker/grunt-plugin-list](http://github.com/manuel-woelker/grunt-plugin-list)