fix(celery): use custom serializer for celery

we use mongo serializer that can take care of ObjectIds and datetimes
Merge remote-tracking branch 'petr/fix-celery-serializer'
fix(io): fix update ingest cmd

it was using superdesk.providers instead of superdesk.io.providers
SD-248 : Handle data uris on server (fix for webcam)
Merge pull request #255 from ioanpocol/master

SD-248 : Handle data uris on server (fix for webcam)