Expose spawned_from_id.

We need a way to get this w/o worrying about it
the job batch that references is still available
in redis.
The error message from redis 2.8 has changed a bit.

Now it includes a `user_script:1234: ` prefix.
Merge pull request #62 from seomoz/expose-spawned-from-id

Expose spawned_from_id.