title: Add a field in newsletter_subscriber Table in magento
body: <p>I wanna make a module by which i can add a field in newsletter_subscriber table in database in magento.
  Although i have added a field in customer table in past but i am unable to enter the field in news letter table. It always give me error "call to undefined function addAttribute".</p>
  
  <p>I have also try to use 
  Mage_Eav_Model_Entity_Setup in config.xml then there is another error 'wrong entity' appear.
  i am using following line to add a field.</p>
  
  <p>$installer->addAttribute('newsletter/subscriber', 'city', array(
          'type'  <code>enter code here</code> => 'varchar',
          'label'     => 'City',
          'visible'   => false,
          'required'  => false
  ));</p>
  
  <p>Please tell me where i am wrong??</p>