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
Well, if I don't it fails if I use my GitHub account in which there is no public email set... and I get `u'email': None,` in the `me.data` returned. Causing a 500 error due to `AttributeError: 'NoneType' object has no attribute 'lower'` ... so I think it is needed.