Fixes reset logic, adds support for ActiveSupport::Cache::Store

The current reset logic didn't work at all, as best I could tell. This commit:
* Fixes the reset logic so that multiple request can hit an endpoint, and rate limiting ever can occur
* Uses seconds since epoch as the cache reset format (e.g. Time#_to_i
* Leaves the rate limit header time formatting the same
* Adds support for using ActiveSupport::Cache::Store (Rails.cache) as the cache
* Adds a section in the readme about the storage mechanism