Responsive Navbar

@jelle since in the pull request on github you mentioned that the responsive navbar has to be added to this repository, I decided to make the existing navbar here responsive from scratch, since the one in the pull request didn't use sass and also generally was a bit different from the one here. This also has the added bonus that the navbar doesn't require any JavaScript, it is pure CSS.

I also moved the whole sass build process into the package.json, since that to me makes installing sass and other future dependencies easier and also feels a bit cleaner, but I can also remove that commit again if you want.

Closes #2