Use Google's OpenID configuration endpoint to fetch provider URLs (suggestion)

[chrisforrence]

Google exposes an OpenID configuration file at the following URL:

https://accounts.google.com/.well-known/openid-configuration

I'm wondering if it would be worth it to reach out to this URL and use it to build up the getBaseAuthorizationUrl, getBaseAccessTokenUrl and getResourceOwnerDetailsUrl strings. Advantage would be having the URLs be always* accurate without needing to deploy a new version of the library (preventing #63), disadvantage would be needing to make an extra HTTP request to fetch said information.

[shadowhand]

@chrisforrence that would be the correct thing to do. The reason I did not make that change was because it is a departure from how previous versions determine the remote URLs.

Happy to review a PR that makes use of it though.