De-/Assign Indicators

* Flask’s SQLAlchemy extension was replaced by pure SQLAlchemy. This allows to easily enable `autoflush=True` and `autocommit=True` to simplify all DB operations.

* *Marshmallow* is a serializer library.

* Relationships between scorecards and indicators are created/selected via *PUT* requests to the `/scorecards/id/indicators/id`.

* Addressed issues #2, #3, #6, #7, #9, #10.