Fixed authentication using GitHub

Fixed authentication using Dropbox as reported in #34 based on example code provided at https://github.com/lepture/flask-oauthlib/blob/master/example/github.py
in ```retrieve_user_from_github()```
``` python
 return create_user_db(
      auth_id,
      response.get('name', response.get('login')),
      response.get('login'),
      response.get('email', ''),
      verified=bool(response.get('email', '')),
    )
```