Database migration tool for SQLAlchemy

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. Documentation and status of Alembic is at http://readthedocs.org/docs/alembic/