Added migrate command for mongodb migrations

This adds a console command to create migration files for mongoyii as the usual `yiic migrate` does. I needed this to spread the EMongoAuthManager updates across all my application instances, but I implemented all writing mongodb commands so you can use it for any migration you may want to do on your databases. I used the `CMigrateCommand` as base and extended all database-related methods, so they work exactly the same.

This also covers the issue #146 as one now can do both, having application logic as well as migration files to handle structural changes.