Adding full off network support.

[bastienwirtz]

Related to #9.
Copy all external resources locally to make it usable full offline.

[fbartels]

Hi, thanks. Looks quite a bit better now. Sometimes a picture amounts for a thousand words:

When switching Chrome to offline after a first normal request then it almost looks good, only the image above the "You're offline bro" is missing.

When now doing a forced reload however (ctrl+shift+r) some more resources go missing. Not quite sure if this can be labeled as expected behaviour, but then it would still be strange that other resources survive this treatment.

[bastienwirtz]

My bad, I forgot the additional font awesome files in the service worker, it should be better now!

[fbartels]

Thanks again. Now the only resources throwing errors are the ?alive request (which I would label as expected) and the favicon.png request.

[bastienwirtz]

Good catch, the favicon was missing too. Thanks to you to contribute and taking some time to test it!
Have a nice day !