Checking events when looking for existing webhooks

This logic only used to check the webhook URL, but the events for that
URL might not be the events this server wants. Now it also ensures all
required events are included, and if not, will create a new webhook.
Allowing new config 'repos' for direct listing.

If the 'repos' config is specified, the 'repos_url' config is ignored,
assuming 'repos' contains properly formatted repository configurations
as if they were fetched from the 'repos_url'. This works well for local
testing and user overrides if a global repo listing isn't needed.
Using first repo value to get gh creds

Before, a repo was hard-coded. Should be fine to get only the first
repo and use its github credentials.
Added 'release' events to webhook registration.
Removes webhooks that don't match event signature.
Removing prints
Removing hooks before creating new ones.
Refined webhook matching logic.
Adding 'tag' event hook logic.
Put a post-tag script in place.
Moved webhook events to config.

Allowing removal of all existing webhooks if githooks: [].
Merge pull request #152 from rhyolight/run-on-tag

Set up post-tag hook for releases.