Added Generic Persistence API and Datatypes

Added a generic persistence API for JobBoards,
Jobs, LogBooks, Workflows, FlowDetails, Tasks,
and TaskDetails. The currently implemented
backends are in-memory and sqlite db (by use
of sqlalchemy). Also implemented generic return
types (listed above) for the generic API.

Change-Id: I05377d09b69ae1acd1f3d7e40ebee01e01c4c432