Give preference to get_session

This gives preference to get_session over model.query.
As a plus, syntax errors such as Unique(User.name, User.email)
are detected sooner because using User.email as session will
fail.
Add test for Flask-SQLAlchemy
Fix InstrumentedAttribute usage in Unique validator
Merge pull request #14 from kvesteri/fix/broken-unique

Fix broken Unique