Make relative paths possible

[jstaerk]

in case someone has her local server set up in a way that /Hello-PWA is not the document root (but e.g. /, so that one would access http://127.0.0.1/HelloPWA/index.html instead of just http://127.0.0.1/index.html ) this allows to run the PWA nevertheless, independent from the path: otherwise the manifest is looked up from the wrong URL, the service worker has the wrong URL and the start_url in the manifest is also incorrect.

[jstaerk]

PS: great tutorial, thanks