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

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.