Logger not created by default for goauth2.App

[pdt256]

This line will fail if no default logger is injected.

https://github.com/inklabs/goauth2/blob/7c2bedaafaca9e4b8cb62be10deda3ba4e7adf6f/goauth2.go#L175

TODO:

• Modify goauth2.New to instantiate a default logger