Some fixes to make mongo-migrate a little bit more flexible

We were using mongo-migrate to run a migration and we like the idea to be able to rollback migrations as well. It worked seamlessly, however we had to make some minor changes to mongo-migrate in order to make it work for us, e.g. without specifying a username and password (since we used it locally). Since I think that these changes are improvements I though it would make sense to integrate them into mongo-migrate officially.

Great tool anyway!