def migrate_up!(level = nil)
      migrations.sort.each do |migration|
        if level.nil?
          migration.perform_up()
        else
          migration.perform_up() if migration.position <= level.to_i
        end
      end
    end
def migrate_up!(level = nil)
      migrations.sort.each do |migration|
        if level.nil?
          migration.perform_up()
        else
          migration.perform_up() if migration.position <= level.to_i
        end
      end
    end
Run all migrations that need to be run. In most cases, this would be called by a
 rake task as part of a larger project, but this provides the ability to run them
 in a script or test.

 has an optional argument 'level' which if supplied, only performs the migrations
 with a position less than or equal to the level.