Log parser infrastructure

This is finally ready for a review.
This patch adds:
- celery setup using rabbitmq as message broker.
- parse-log task to run the parser on a job's log references
- trigger a parse-log task once a log reference is created 
It refers to https://www.pivotaltracker.com/story/show/49080725

Task routing based on the job result is still missing, I want to do it in another branch together with the job result mapping