Generic single-database configuration.

Alembis generates migration scripts by comparing the data model against the
database. The migration only works if the model is newer as the database.

How to use:

1. Adding new modul

* Backup the database
* Add new modul -> Database will be changed
* Restore database -> Revert changes in the database, leave changes in the
  python model
* Run "alembic upgrade head" to mark the table having all all migrations
  included
* Run "alembic revision --autogenerate -m "Added file table"" to generate a
  migration script which has the migration from the newer model to the older
  database

2. Manual changes in the model

* Run "alembic revision --autogenerate -m "Added file table"" to generate a
  migration script which has the migration from the newer model to the older
  database
