Show a configurable list of model scopes in list view

This allows to display a configurable scoped list view.

Use cases: order state in ecommerce, list of new models that need to be moderated, etc

## Usage:

    list do
      scopes [:not_cart, :cart, nil]
    end

First will be the default, nil means no scope (also it is possible to disable non-scoped index)

## How it is displayed

![w64](https://f.cloud.github.com/assets/1041407/1712639/248c4ebe-6179-11e3-9ef0-6c63f3c5386d.png)

When no scopes are configured, tabs are not shown.

I18n key is admin.scopes.scope name