Expiring restriction keys after their effective period has finished.

- This helps keeping redis resque namespace more clean, by using redis expire command to set an expiration for the restriction key by the same period of its effectiveness.
- For example, if a job is restricted by minute, the first time that restriction key is set it will be marked to expire in 60 seconds, for a job that is restricted by hour it will be marked to expire after an hour, and so on.
Merge pull request #6 from vicentemundim/expire_restriction_keys

Expiring restriction keys after their effective period has finished.