Attach entity_reference selection handler on a form element without creating field/instance

This is an attempt to solve this feature request. This patch only tackles the textfield form element.
The items bellow describes the list of accepted parameters for the '#entityreference' property:

type: (optional, default: tags) [single|tags]
handler: (optional, default: base) the entity_reference handler.
target_type: the target entity type.
handler_settings: (optional) An array of handler settings such as target_bundles, sort and auto_create.
This is an exemple of implementation:

    $form['returns'] = array(
      '#type' => 'textfield',
      '#title' => t("order's returns"),
      '#size' => 80,
      '#maxlength' => 160,
      '#required' => FALSE,
      '#entityreference' => array(
        'target_type' => 'commerce_order',
        'handler' => 'base',
        'type' => 'single',
        'handler_settings' => array(
          'target_bundles' => array('commerce_return' => 'commerce_return'),
          'sort' => array()
        )
      )
    );