Admin items presenters

Hi @joneslee85 

I added the presenter as you did in the normal Items Controller -> this uses it for @items, and it was fine.

But I'm not sure about extending it for @item as well.

I tried to set it as item = Item.find(param[:id]) in the presenter, but that wasn't working.

I have watched this http://railscasts.com/episodes/287-presenters-from-scratch -> is this something I should be learning from? 
talk to Model is the responsibility of controller not View, so you should call to Model like below:

```
items = Item.all
@presenter = ItemsPresenter.new(items)
```

in your presenter

```

def initialize(items)
  @items = items
end

attr_reader :items
```