using SOCIAL_APP_URL to provide a proper callback url 

I ran in to the problem of the callback URLs having localhost:5000 (`request.url_root[:-1]`) in them.  Without having to use Flask's SERVER_NAME options, I decided to using the SOCIAL_APP_URL variable that was added to the app.config by default.

I changed the behavior such that SOCIAL_APP_URL defaults to None.  In those cases, request.url_root[:-1] is used for generating the callback urls.