Adds an optional TTL to limit cache time

We have external services that update cached models without going
through rails (not a recommended design), and found that the best
way to avoid serving cached results from record-cache was to add a
TTL to these models, so the data would expire after a set time.
Merge branch 'add_ttl_option' of https://github.com/justintv/record-cache into justintv-add_ttl_option
Merge branch 'justintv-add_ttl_option'