Cleanup and force `grunt server:dist` to use production resources.

* Remove the `watch` task, we weren't using it.
* Update production.json's `static_root` to be `static_directory` which forces `grunt server:dist` to use production files.
* Hook `npm start` to `grunt server`
Merge pull request #137 from mozilla/more-gruntfile-cleanup

Cleanup and force `grunt server:dist` to use production resources.