AuthScheme and improved example

[SherClockHolmes]

Fixes for https://github.com/SherClockHolmes/webpush-go/issues/82, https://github.com/SherClockHolmes/webpush-go/issues/72

Updated the library to support both the original and modern VAPID authentication schemes (vapid and webpush) to handle browser inconsistencies. A new AuthScheme option has been added to the Options struct to select the desired scheme, defaulting to vapid. The example application has also been updated to handle the management for vapid keys automatically.