Added Padrino support for Sidekiq and made it framework independent

Hi @mperham,

I believe Sidekiq is a great platform and I reckon it's time we make Padrino a first class citizen on it :), that's why I've implemented its support along with the possibility to add more frameworks in the future.

Now you can just run `bundle exec sidekiq` on your Padrino app and it will work as well as it does on Rails! :)

There's still one catch and it has to do with ActiveRecord methods, at the moment you have to send them yourself in `config/boot.rb` as follows:

```
Padrino.after_load do
  ::ActiveRecord::Base.send(:include, Sidekiq::Extensions::ActiveRecord)
end
```

You'll find a demo app with the same functionality than the Rails app has in `/demo-apps`.

Let me know what you think. :)

Thanks,
Darío