deposit: fix for validation before import.

* Improves TaskManager to use deferred objects better.

* Adds validate() function making a server-side validation
  in background.

Signed-off-by: Kamil Neczaj <kamil.neczaj@cern.ch>
deposit: duplicate validator with prod inspirehep

* Queries inspirehep.net to check if a submited publication is
  already in the production db.

* Introduces PRODUCTION_MODE variable which enables cooperation with
  inspirehep.net server.

* NOTE Set PRODUCTION_MODE=True in your config to use the validator.

Signed-off-by: Kamil Neczaj <kamil.neczaj@cern.ch>
deposit: import fields validation improvements

* Raises StopValidation when syntax of a field is not valid.

Signed-off-by: Kamil Neczaj <kamil.neczaj@cern.ch>