Error in authenticateLocalUser() method

[christopherl]

Great helper class, thanks for that! Noticed a little bug in the authenticateLocalUser method, though: As apple states in their documentation, you should always check the authenticated-property on the local player, instead of relying on an error.

Also, if the view is not nil, there should be code to present the given view controller, so the user sees a login screen.

Thanks again!
Chris