Minor update to migration_done

Hi, I've added direction to Adapter:migration_done, so now 'down' migrations deleting records from `schema_migrations` where version = Tag.
Otherwise
 1. we'll have number of records with version = Tag
 2. up migrations in list won't be run again, even if needed.

I've also added get_migrations_table/1, migration_done/3 into boss_db_adapter_mysql.erl

One more thing, should we rename column 'version' into tag or something similar?