Clear site cache when WP Core is upgraded, or an active theme or plugin

Resolves websharks/quick-cache#327
Clear site cache when WordPress Reading Settings are updated

Resolves websharks/quick-cache#223
Clarify auto_clear_cache() Dashboard message

Resolves websharks/quick-cache#328
Enhancing `upgrader_process_complete()`. See: https://github.com/websharks/quick-cache/pull/330
Use $upgrader_instance to check plugin state with a single-plugin update
Build a reference to \Plugin_Upgrader_Skin for IDEs
Merge branch 'feature/327' into 000000-dev
Move old changelog entries to separate file to decrease size of readme

 The readme.txt size was causing issues with the readme parser on
 WordPress.org. Old entries will periodically be moved to the new
 CHANGELOG file.
Add Pro Installation section to readme.txt
Merge branch '000000-dev' into feature/55