One bug, one feature

**Very small modifications, which basically brings two things with them:**
1. One fixes a bug, when trying to migrate to version 0, therefore no migration files are available on the file system.
2. One adds a small feature, when you don't pass the -m parameter, now db-migrate is able to migrate down, it it's the case (using the database or course) to the last migration file available to the file system.