Accept timedelta for ttl

[allen0099]

Problem

ttl must be an int number of seconds everywhere. @cache(ttl=timedelta(minutes=5)) raises CacheXError: ttl must be an int number of seconds, got timedelta, and the backends and CacheManager raise TypeError.

Proposal

Accept datetime.timedelta wherever a ttl (or stale_ttl) is taken, converting it to whole seconds, and reject values that are not whole seconds or that are out of range, as today. Keep bool/float rejected.