Added _updated and _error attributes to jobs.

Hey Manuel,

We're working on a small web interface to admin bull jobs (like kue). It will be a totally independent module so bull code can stay minimal. We just need a few more things in bull:

- `job._updated`: last time the job was moved to a different state.
- `job._error`: error message of failed jobs
- `job.retry()`: move the job back to the "wait" queue.