Added tests to describe new "expiration" feature
 - Added LRU_TTL cache engine that discards expired values. TTL is defined as time.Duration, not just int64, just to avoid confusion (is it ms? or ns? or something-else-ns?).
 - Switched dependant groupcache subpackages from "github" to local.
 - Switched groupcache to use lru_ttl instead of just lru
Merge pull request #1 from elgris/lru_ttl

TTL cache engine implemented