Support multiple databases in migrations

ActiveRecord natively supports multiple database connections. There's no reason for migrations not to do the same.

Not to mess things up, migrations for default database should still reside in db/migrate, while others should reside in db/migrate/database_config_name_without_environment_name.

Also, db:migrate should support option for specifying which database to run migrations on.

Directory layout example:

db/migrate db/migrate/second

Corresponding db config:

### config/database.yml
development:
  adapter: sqlite3
  database: main_development

test:
  adapter: sqlite3
  database: main_test

production:
  adapter: sqlite3
  database: main_production

second_development:
  adapter: postgresql
  database: second_development

second_test:
  adapter: postgresql
  database: second_test

second_production:
  adapter: postgresql
  database: second_production