master add SCOPE_SEPARATOR to DisqusOAuth2

[cansadadeserfeliz]

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.

[omab]

Thanks!