Make the Mounter support the Rack application

I'd like to allow to mount the Rack application to Padrino apps.
If we want to mount the rack application to the main application currently, we have to define some class methods on the mounted application class.
It's very troublesome.

```ruby
class API < Grape::API
  class << self
    # some class methods here
  end
end
```

This commit makes the mounter smoothly support the Rack application.
The apps.rb code would be something like:

```ruby
# config/apps.rb

Padrino.mount('SinatraApp', app_file: Padrino.root('sinatra_app/app.rb'))
Padrino.mount('Sample::API', app_file: Padrino.root('api/app.rb'))
```

@padrino/core-members What do you think?