Modified country_select to permit passing :object as an option.

I'm faced with the following scenario:

* Using Formtastic to generate forms
* Using a nested form:
  * form is created for an Order model object
  * sub-form is created for an Address model object
  * using `form.input :as => :country` as the form-field for the Address country

The Address object was not being passed through the whole field generation stack so therefore the previously-set value for the Address country was not being pre-selected in the country_select.

To fix this, I've added the ability for `country_select` to pull the `object` from the `args` in the same way that `subregion_select` does.  That way I can pass the object manually from my sub-form, e.g.:

    = subform.input :country_code, :as => :country, 
                    :priority_countries => %w[US CA], 
                    :object => subform.object