support dynamic repositories

[nicolai86]

Right now the bot needs to be configured with a list of repositories it supports - moving forward I'd be really interested in extending this to an arbitrary number of repositories, e.g. by using Github user/ organization wide webhooks, so the bot get's events for ALL repositories.

One way to handle this would be to dynamically create caches for the repositories, but to do this we'd need to refactor the entire event handling to support parallel repo & branch operations - this issue is side stepped ATM via separate HTTP handlers per repo.