title: iOS FB-sdk 3.1 - login with different username
body: <p>I added test-users to my FB-app on FB and now I want to login to FB from my iPHone-app using the test user's credentials.
  But I used my own username already and now FB only presents the «allow permissions» screen and always skips the login screen.</p>
  
  <p>How do I tell my app to forget previous logins and use new credentials?</p>
  
  <p>[Edit]</p>
  
  <p>This is on the simulator, not an actual device...</p>