Automatic refresh of Authorization Code Flow Tokens in long-running Applications (#428)

* auto-refresh user token

* example for a long-running user-request app

* wrap long lines

* combine duplicate code into _refresh_token_if_expired method

* add changelog entry

CHANGELOG.md

@@ -6,6 +6,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+ - Auto-Refresh token in long-running apps when using Authorization Code Flow
+
 ## [2.7.1] - 2020-01-20
 
 ### Changed