Update task status on callback (needs #981)

In order to show the task status (#939), we have to know the status of a job. The datapusher sends the status of a job if it fails or succeeds. 

Datapusher: https://github.com/okfn/datapusher

https://github.com/okfn/ckan/wiki/Spec:-DataStore-and-FileStore-Consolidation