Base migration class?
Take a look at the example migration for beer and wine that ships with Migrate in the 'migrate_example' folder.

You will need to register your your migration and handler classes explicitly with hook_migrate_api(). See 'migrate_example.migrate.inc' inside 'migrate_example'.

However, you don't need to explicitly register abstract intermediate classes that hold common info for other migrations you're implementing. See the abstract class 'BasicExampleMigration', inside 'beer.inc' as an example.