Class registration for Migrate 2.5 or later

It's a long story, but due to fatal errors in certain environments triggered by Migrate's automated registration of migration and handler classes, as of Migrate 2.5 it will be possible to disable auto-registration, and any module implementing migration or handler classes should explicitly register them via hook_migrate_api() to continue working when auto-registration is disabled. Adding this registration will have no effect with Migrate 2.4 or earlier.