TTL for jobs

Time to live for jobs. I had a case where I wanted to not process jobs older than a certain date. Easiest solution was to add a ttl attribute to the job and if set expire the job in redis. The job will still be in the list of jobs but rq will just discard it. A job had already a `get_ttl` method that returned the result ttl. I renamed it to `get_result_ttl` and added a `get_ttl` that returns the job ttl.
Might be worth using the existing `cleanup()` method to implement this TTL.