Adding multiple database support for determining migration location with ActiveRecord 4.0.0

This commit overrides `ActiveRecord::Generators::MigrationGenerator#create_migration_file`
in order to allow the setting of the migration directory via the
migration generation mechanism.

The benefit of this is realized when generating migrations for
specific databases that have different migration folders specified in
the `.database_name.standalone_migrations` files.

* Added `config/database.yml` to list of default config directories
  since it tends to be a very common place to put database
  configuration files.

* Added StandaloneMigrations::Configurator#db_dir to set the location
  of the database directory for ActiveRecord::Tasks::DatabaseTasks