Update CExistValidator.php

Adds an ability to use CExistValidator for arrays, for example: for a list of checkboxes.

To do this, you need to specify the attribute "multiple" ('multiple' => true) in the returned array of function rules():

public function rules()
{
return array(
array('attribute list', 'exist', 'attributeName' => 'some attribute name', 'className' => 'some class name', 'multiple' => true),
)