Fix removing application auth tokens
Encode all generated error messages
Add decorator for new methods requiring app creds
Apply decorator to appropriate functions
Add GitHubSession#retrieve_client_credentials

- Convenience method for revoking authorizations
Fix sketch of requires_app_credentials
Rework revoke_authorization(s) with new methods
Doc-strings for tests

- ALso remove the unnecessary pieces of the tests
Merge branch 'pr/213' into develop