add a clean script which removes npm installed libs

This script just removes library files installed by npm install so that you can do a clean
npm install
for gh-pages branch.

BTW: I realized that you can do a "git clean -df" for github-refactor-oerpub. However this script is useful for gh-pages and merged into this branch. So decide Izak if you want to add it also to github-refactor-oerpub.