Alembic is a new database migrations tool, written by the author of
SQLAlchemy. A migrations tool offers the following functionality:

- Can emit ALTER statements to a database in order to change the structure of
  tables and other constructs
- Provides a system whereby "migration scripts" may be constructed; each
  script indicates a particular series of steps that can "upgrade" a target
  database to a new version, and optionally a series of steps that can
  "downgrade" similarly, doing the same steps in reverse.
- Allows the scripts to execute in some sequential manner.
