support polling where webhooks are unavailable

[nicolai86]

webhooks can only be used when you have admin access to the repo in question. sometimes you want to act even when you are a collaborator.

by making the public-dns flag optional we should add a fallback mechanism which regularly polls the repo in question to execute the same behaviour. this way the bot would works in either case.

also, it pays on the long term goal to be not github dependent.