Added a migration script generator script.
Any changes to DB structure are to be regarded as a migration. Our first migration takes us from an empty DB to one that can store users.
SQLAlchemy-migrate compares the structure of the DB against the structure of our models. The differences are recorded as a migration script ('script') inside the migration repo.
The migration script knows how to apply or undo a migration - allowing upgrade or downgrade of DB format.
Note: Don't rename existing fields, add or delete them, change their types if needed.