title: Extend sfGuardUser : adding "phone number" field
body: <p>DOCTRINE 1.2 // SYMFONY 1.4</p>
  
  <p>Hi!
  in my website, am using the <code>sfGuardPlugin</code>'s tables with the other tables in order to set an alarm system, the app gets to the users mails in order to get the mails and send the email, 
  now i need also a phone number for sending sms !
  all i need is an extra field ("<code>phone</code>") in the <code>sfGuardUser</code> 'table,
  i have been wondering if i could do it with "<code>inheritence</code>" but am stuck in the type (simple, concrete? ...)
  any idea?</p>