Add select and Multiselect option to System/Config/Form/Field/Array in backend

Add extra field types to the Backend Configuration Field type array:

![screen shot 2014-02-01 at 12 13 44](https://f.cloud.github.com/assets/6409584/2057492/7caab1ce-8b33-11e3-92ac-7ee95bb4479c.png)

Usage example:

In adminhtml:
```
<item_fields translate="label comment">
    <label>Attributes</label>
    <frontend_model>core/system_config_form_item_fields</frontend_model>
    <backend_model>adminhtml/system_config_backend_serialized_array</backend_model>
    <sort_order>10</sort_order>
    <show_in_default>1</show_in_default>
    <show_in_website>1</show_in_website>
    <show_in_store>1</show_in_store>
</item_fields>
```
Class Mage_Core_Block_System_Config_Form_Item_Fields:

    public function __construct()
    {
        $this->addColumn('select', array(
            'label' => Mage::helper('core')->__('Select'),
            'type' => 'select',
            'options' => Mage::getModel('adminhtml/system_config_source_yesno')->toArray(),
            'style' => 'width:100px'
        ));

        $this->addColumn('multiselect', array(
            'label' => Mage::helper('core')->__('Multiselect'),
            'type' => 'multiselect',
            'options' => Mage::getModel('adminhtml/system_config_source_yesno')->toArray(),
            'style' => 'width:100px'
        ));

        $this->addColumn('text', array(
            'label' => Mage::helper('core')->__('text'),
            'style' => 'width:100px'
        ));

        $this->_addAfter = true;
        $this->_addButtonLabel = Mage::helper('core')->__('Add new attribute');
        $this->setTemplate('system/config/form/field/array.phtml');
        parent::__construct();
    }