add popup view

the parameter will be like ```?show_popup=[cardname]```
Good stuff.  I will merge as is, but I suggest these small tweaks:

1. tiny one: you can just use params[:show_popup] (you don't need Card::Env)
2. I like avoiding "return" wherever possible, so you could just use an else
3. (most important one) usually, when creating a format object within another format object, the best practice is to use "subformat", not "format", because this will then preserve a format hierarchy with parents, name contexts, etc.

so I'm picturing something like:

```
view :popup do |args|
  card_name = args[:show_popup]||Card::Env.params[:show_popup]
  after_card = Card[card_name]
  if !after_card
     Rails.logger.info "Expect #{card_name} exist"
  else
     "<div class='popup-window'>#{ after_card.subformat( :format=>:html).render_core } </div>"
  end
end
```