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.

The source code for the heroku app is available at [manuel-woelker/grunt-plugin-list](http://github.com/manuel-woelker/grunt-plugin-list)