Add Lita::Scheduler

here is an usage example:

```ruby
module Lita
  module Schedulers
    class Sample < Scheduler
      every '5s',        :job1
      cron  '* * * * *', :job2

      def job1
        send_message(room: 'foo',  message: Time.now.to_s)
      end

      def job2
        send_message(user: 'bar',  message: 'hi!')
      end
    end
    Lita.register_scheduler(Sample)
  end
end
```

- using [jmettraux/rufus-scheduler](https://github.com/jmettraux/rufus-scheduler) inside