On v3, please provide only secret_key OR public_key.

Providing ``public_key`` AND ``secret_key`` leads to trouble.
See: https://groups.google.com/d/msg/disqus-dev/9HBAftO0jr0/gD_j-rWdSqsJ

Question: should we get rid of public_key mentions on the whole codebase?
You are right.

I cannot find out an entrypoint that does not require at least the ``public_key``, but [here](https://disqus.com/api/docs/requests/) is said that Disqus "recommend using SSL for all requests to the API, but it is not required for **anonymous requests**.".

Question is: what "anonymous requests" means? If nothing can be hit without public or secret key, is safe to raise an exception for this case.

If someone can confirm that, I will happily update the PR to raise an exception for no key provided, or at least put a warning.