master  add SCOPE_SEPARATOR to DisqusOAuth2

Adding ``SCOPE_SEPARATOR`` to ``social.backends.disqus.DisqusOAuth2``.

According to Disqus OAuth 2 documentation it can receive scope parameters separated by comma:

    Location: https://disqus.com/api/oauth/2.0/authorize/?
    client_id=PUBLIC_KEY&
    scope=read,write&
    response_type=code&
    redirect_uri=http://www.example.com/oauth_redirect

https://disqus.com/api/docs/auth/

So, for example, one can add ``email`` to scope parameters end obtain user's email as extra data.