retrieve and populate email field when signing up with openid

adopted def ask_openid of current django_authopenid
https://bitbucket.org/benoitc/django-authopenid/src/1363e3e3eb9b62b0dbb104cde51ba12eb7bfacd4/django_authopenid/views.py?at=trunk

http://askbot.org/en/question/10229/facebook-sign-in-can-screen-name-and-email-be-auto-populated/