Fixed authentication using StackOverflow (redone)

Applied #46 to the current gae-init-auth code base; see the original PR for details.
Hmm, what exactly do you suggest?  I recall that the `items` could be empty or non-existant, when signing in with a non-SO account (using another SE account... despite the argument in the request for SO).

Should I go for:

```
  if not me.data.get('items') or not me.data['items'][0]:
    return 'Missing account details for Stack Overflow.'
```