Add some missing interfaces; alter handling of missing `public_key`

[strix-technica]

Add three endpoints missing from interfaces.json added (posts.update, forums.addModerator and forums.removeModerator);

Somewhat cosmetic change: Some API calls (notably posts.create with certain args like ip_address) will not work as expected if both public_key and secret_key are passed, so alter the check in disqusapi.DisqusAPI.__init__() so that it doesn't complain when only secret key is provided.

[mattrobenolt]

@strix-technica Seems that you've broken tests. :)

[strix-technica]

Well, that was embarrassing. I've fixed it, tested it with Travis on my own fork, and it appears to be good. (And if I read this right, that fix automatically got included in the pull request and Travis has already run and passed it).