allow plugins to add cron jobs

fixes #1015

Example:
```
module ConcertoCusotmPlugin
  class Engine < ::Rails::Engine
    def plugin_info(plugin_info_class)
      @plugin_info ||= plugin_info_class.new do
        perform_job_every 1.seconds, "Tick" do
          puts "tick"
        end
      end
    end
  end
end