Build project with node 9 & npm 5

[rjoonas]

Found out that building the project works with node 9 (with deprecation warnings) but is broken on node >= 10. So I made the following improvements:

• Add .nvmrc so nvm users can easily install and use right version of node & npm (by running nvm install in the project directory)
• node 9 ships with npm 5 which generates lockfiles -> added package-lock.json autogenerated by npm
• Add .npmignore so that .nvmrc is not published to npm registry.