workflows: overhaul update

* Adds more documentation.

* Fixes some sphinx errors in documentation.

* Changes imports to relives one when possible.

* Adds a new way to handle errors, to allow easy reading.

* Removes obfuscations of exceptions.

* Clarifies exceptions.

* Updates the way exception unicodes are handling.

* Fixes the behaviour of stop_on_error.

* Adds python 3 compatibility when possible.

* Adds a method to clean extra_data in a Wfe.

* Fixes problems with python 2.6 loggers.

* Adds reliable way to handle types for BibworkflowObject.

* Adds a method to get widget message easily.

* Adds an abstraction method for bibfield conversion in
  workflow tasks.

* Adds more tests.

* Adds start test procedure.

* Fixes bad sorting instructions in bibholdingpen.

* Improves the way of restarting workflows.

* Simplifies celery worker.

* Reorganizes tree of the module.

* Tasks cleaning and optimizations.

Signed-off-by: Guillaume Lastecoueres <guillaume.lastecoueres@cern.ch>