Add the job's PK to the Error model

Closes #6
Add specific Exception
Allow jobs to be requeued in case of failure

Closes #7
New worker argument to requeue failing jobs

"requeue_times", default to 0

cf #7
By default decrease priority of a requeued job (it's a worker attribute)

"requeue_priority_delta=-1"

cf #7
Bump version to 0.0.5 and add CHANGELOG file