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
lines 1064-1068
``` python
  emails = [
       email for email in response.get('emails', []) if email.get('handle')]
   emails.sort(key=lambda e: e.get('primary', False))
 
   email = emails[0]['handle'] if emails else ''
```