Documentation fixes and refactor

[darrenklein]

Thank you so much for creating this very helpful example repo! While using it to help me in my own project development, I spotted a few small fixes that I think would benefit this project.

• replaces use Guardian.Token.Jwt.SecretFetcher with @behaviour Guardian.Token.Jwt.SecretFetcher, since the example uses a custom implementation of fetch_signing_secret and fetch_verifying_secret rather than the default implementations use would provide.
• updates the documentation to match that, including a fix for a reference to :debug_guardian
• other small refactors and cleanup

Again, many thanks - your example helped me out a lot, I hope these contributions are beneficial.