OAIHARVEST: Global improvement.

* DBlayer refactored for oaiharvest part. Now using sqlalchemy models.
* OaiHARVEST table schema changed to add workflow options and remove
  frequency one.
* Now harvesting with --workflow or -r --workflow or only -r options are
  working.
* Delete the frequency parameter.
* Generic workflow is now taking parameter of the given repository.
* Multi harvesting possibilities.
* Bibsched communication addition.
* Fix a bug about multi workflow to run in one.
* Fix a bug about a misused of Bibfield