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?
Well, this doesn't warn them if they omit both. Not sure if it is necessary, just pointing out.