Prevent against 'created_at' field being None.
Attach job ID when unpickling fails in .dequeue().

This makes the behaviour consistent with .dequeue_any().
Requeuing a job should set its status back to Queued.
Merge branch 'master' of github.com:selwin/rq
Merge pull request #177 from selwin/master

FailedQueue.requeue should set job status back to Queued
Merge branch 'master' into scheduler-integration
Merge branch 'master' of git://github.com/nvie/rq into scheduler-integration
Added queue.schedule_at and queue.schedule_in methods.