Improve documentation by describing what is expected of a class implementing CacheInterface. (#21, #22, #23, #27 by @WyriHaximus)

Implemented (optional) Least Recently Used (LRU) cache algorithm for ArrayCache. (#26 by @clue)

Added support for cache expiration (TTL). (#29 by @clue and @WyriHaximus)

Renamed remove to delete making it more in line with PSR-16. (#30 by @clue)