How to create multi select attribute for customer using install script?

Question

I'm following many articles but i'm not able create multi select attribute. and i'm getting following error.

a:5:{i:0;s:50:"Source model "" not found for attribute "exinent1"";i:1;s:3008:"#0 C:\wamp\www\magento8\app\code\core.

and my script is:

<?php
$installer = $this;
$installer->startSetup();

$setup = new Mage_Eav_Model_Entity_Setup('core_setup');

$entityTypeId     = $setup->getEntityTypeId('customer');
$attributeSetId   = $setup->getDefaultAttributeSetId($entityTypeId);
$attributeGroupId = $setup->getDefaultAttributeGroupId($entityTypeId, $attributeSetId);

$setup->addAttribute('customer', 'exinent1', array(
    'input'         => 'multiselect', 
    'type'          => 'varchar', 
    'backend'       => 'eav/entity_attribute_backend_array',
    'label'         => 'exinent1',
    'visible'       => 1,
    'required'      => 0,
    'user_defined'  => 1,
    'option'        => array (
                'value' => array('optionone' => array('First Option'),
                         'optiontwo' => array('Second Option'),
                         'optionthree' => array('Third Option'),
                         )
                        ),

));

$setup->addAttributeToGroup(
 $entityTypeId,
 $attributeSetId,
 $attributeGroupId,
 'exinent1',
 '100'
);

$oAttribute = Mage::getSingleton('eav/config')->getAttribute('customer', 'exinent1');
$oAttribute->setData('used_in_forms', array('adminhtml_customer')); 
$oAttribute->save();

$setup->endSetup();

and config.xml is:

<?xml version="1.0"?>
<config>
    <modules>
        <Exinent_Cusatt>
            <version>3.1.5.6</version>
        </Exinent_Cusatt>
    </modules>
    <global>
    <resources>
    <exinent_cusatt_setup>
        <setup>
            <module>Exinent_Cusatt</module>
        </setup>
    </exinent_cusatt_setup>
</resources>
    </global>
</config>