Fixed a bug when tried to do down migrations.

Fixed a bug when tried to do down migrations but no migration files were
available, even though I was using the database registered migrations
only.

Example:
1. Make sure you have only one migration executed so far. So you should
have one migration file and one migration entry in the database.

2. Delete the migration file.

3. Try to migrate to vesion 0. You will reiceve a 'no migration files
found' exeception.

I cleaned up the thigs a little bit and now some operations are done
only if it's the case, like up_migrations or force_files.
Feature: autodetecting the last migration and if it should do migration_up or migration_down