Add getter / setter for OAuthToken createdAt property

Hi there,

I need to persist the OAuthToken in database, and I'd like to only persist `rawToken` information to avoid unnecessary user serialization.

I need to do so to be able to re-inject a createdAt value in a freshly created OAuthToken object.

Can I also create another PR where variables are `protected` and not `private` to be able to extend more easily the OAuthToken class?

Thanks a lot

Best