Support for multiple tabs in Resources#show action

Hi, I've added multiple column support for the show action. I need this for a project because we have a lot of data about the user, his parents, etc and they need a way to look each data isolated.

The final result is showed : http://img88.imageshack.us/i/screenshotwxp.png/

Tabs are  named and configured nesting them in the show fields list :

```
fields:
    show:
      pasajero: email, created_at
      padre: father_name, father_workplace, father_email, father_phone, father_cellphone
      madre: mother_name, mother_workplace, mother_email, mother_phone, mother_cellphone
      contrato: contract_accepted_on, price_with_currency
```

If you want to short the tabs, you can named as :

```
fields:
    show:
      01_pasajero: email, created_at
      02_padre: father_name, father_workplace, father_email, father_phone, father_cellphone
      03_madre: mother_name, mother_workplace, mother_email, mother_phone, mother_cellphone
      04_contrato: contract_accepted_on, price_with_currency
```

And then set the name using the locale name :

```
  admin:
    tabs:
      user:
        01_pasajero: 'Pasajero'
        02_padre: 'Datos Padre'
        03_madre: 'Datos Madre'
        04_contrato: 'Contrato'
```

The patch come with some tests, but let my know if they are not enough. I'll try to improve the code in the next days, but for now, I think is a very decent "hack" :). Comments are welcome.