Feature/queue indicator calculations

Adds a new endpoint to create an IndicatorJob object and
submits a job to an indicators queue for celery.

Celery processes one job a time, with a timeout currently
set to 10 minutes for processing a job, retrying 3 times
possibly.

Celery tasks were split into 2 queues:
  1. datasources - used for processing datasource imports (osm, gtfs, etc)
     because these can be done concurrently without too much overhead
  2. indicators - used for calculating indicators; cannot be done concurrently
     so limited to one celery worker with timeouts and retries set
I'm not sure whether this will be workable since we have to interface with Scala, but Celery has its own set of job states that we could potentially make use of: http://celery.readthedocs.org/en/latest/reference/celery.states.html and http://celery.readthedocs.org/en/latest/reference/celery.result.html