Use bower to manage frontend dependencies.

Also remove modernizr in the process, since we're not using it anywhere.

To install new frontend dependencies now, simply do a `bower install
<lib> --save`. All existing dependencies can be fetched with a `bower
install`.

Closes #25.
Use relative URLs in stylesheet instead of absolute.

This makes sure they also work in devserver, and will work for
any version on the server.

Closes #26.