Update py-flask-sqlalchemy to 2.0.

Version 2.0
-----------

Released on August 29th 2014, codename Bohrium

- Changed how the builtin signals are subscribed to skip non
  Flask-SQLAlchemy sessions.  This will also fix the attribute error about
  model changes not existing.
- Added a way to control how signals for model modifications are tracked.
- Made the ``SignallingSession`` a public interface and added a hook
  for customizing session creation.
- If the ``bind`` parameter is given to the signalling session it will no
  longer cause an error that a parameter is given twice.
- Added working table reflection support.
- Enabled autoflush by default.
- Consider ``SQLALCHEMY_COMMIT_ON_TEARDOWN`` harmful and remove from docs.

(kleink)