Allow use of autoincrementing id fields for Postgres and MySQL

Provide an option for Postgres and MySQL configurations to use autoincrementing (numeric) row IDs rather than UUIDs.

Configurable (defaults off) via the database adapter configuration object (e.g. `geddy.config.db`). Set `autoincrement = true` to activate. Configuration affects all tables of the adapter in question.

Not taking in account table creation yet. Will support if needed.