Accept the page_build event and refactor a ton.

* Add `script/console` to let you mess around with things
* Add `src/event-types/all.coffee` with all the event types' handlers in one place. Should probably separate out into individual files soon.
* Strip out unnecessary non-hubot things from `src/github-repo-event-notifier.coffee`
* Use the `X-GitHub-Event` request header to determine which handler to use
* Print warning to the console if `HUBOT_GITHUB_EVENT_NOTIFIER_TYPES` isn't set rather than failing silently

Fixes #13.