Updated Disqus provider (now works fine)

Disqus was not working due to URLs being old and their APIs being weird.

I have:
- added a trailing slash to the token_url in order to correctly receive the access_token from Disqus;
- removed the curl_header part as it was not necessary;
- added 2 params (api_key pointing to client_id and api_secret pointing to client_secret) which are required for Disqus to make API calls;
- serialized the incoming data (some may still be missing, I tested this using my incomplete Disqus account).

Hope this helps. Keep up this awesome work!